Preface

As you're no doubt aware, Undyne the Undying is a pretty difficult boss. She has fast and complicated attacks, which require a large amount of memorization and skill. There's no substitute for being good at the game, but 'git gud' and 'bash your head against the wall until you get it' aren't very helpful in terms of advice. That's where this guide comes in.

This guide assumes you have played Undertale's True Pacifist route to completion, and have started a Genocide/No Mercy route, just before encountering Undyne the Undying and getting your ass kicked. This guide limits spoilers for the remainder of the No Mercy route, but it does have a section related to speedrunning, found at the bottom of the page.

The following sub-sections will be organized by their relevence to the average player.

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Equipment

Of all of the equipment available up to Undyne the Undying, four stand out among the rest:

As you can see, the Ballet Shoes and Old Tutu provide better attack and defense, but the Torn Notebook and Cloudy Glasses provide extra invincibility frames.

You may be wondering what these stats do in practice. Well, I'll tell you! The following assume an LV of 10. Note that Undyne the Undying has 23000 HP:

AT:

Ballet Shoes:

I can't accurately guess how good you'll be with the Ballet Shoes, but by my guess, assuming you don't use any turns to heal, it'll probably take about ~18 turns (you'll see ~17 attacks). If you're good with the Ballet Shoes, you can expect to see about 15-16 attacks.

Torn Notebook:

Again, I can't be sure of your skill level, but I think it'll probably take about ~21 turns (you'll see ~20 attacks). If you're good with the Torn Notebook, you can expect to see 17-18 attacks.

DF:

The following information assumes an LV of 10.

Old Tutu:

8-12 damage per hit, with an average of ~9.75 damage (you'll die after taking ~5.75 hits).

Cloudy Glasses:

8-13 damage per hit, with an average of ~10.5 damage (you'l die after taking ~5.33 hits).

INV:

'INV' is is a statistic that determines how long you're invincible after being hit by an attack.
Each point of INV is one frame that you're invincible. As Undetale runs at 30 FPS, each point of INV is 1/30th of a second of invincibility.

You normally have 30 INV, but against Undyne the Undying, you have one less INV per LV, meaning you'll probably have about ~20 INV.
One might assume that the Torn Notebook and Cloudy Glasses would give 6 and 9 INV, but, that's actually incorrect - they give 15 and 30 INV respectively - a very potent buff that will save your ass.
Wearing both will give you 45 INV, which will over triple your INV STAT.
The Cloudy Glasses alone will still give you over twice the INV, so wearing them alone is still more than worth it.

Equipment Closing Thoughts

Having either the Torn Notebook or Cloudy Glasses adds loads of i-frames, and having both adds even more, so the extra INV is almost always worth it in some capacity. With that said, the extra attacks you'll have to endure by using the Torn Notebook instead of the Ballet Shoes can get you killed, especially given how much harder the attacks start getting. Since the Cloudy Glasses add more INV anyway, if you're gonna use only one INV boosting item, it should probably be the Cloudy Glasses.

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Consumables

Here's two lists of all of the relevant consumables, in order of HP gained, separated by whether or not you can backtrack to get them:



Which of these are the most useful? Well, before we talk about that, let's first discuss the Pie and Snowman Pieces.

As you've probably guessed, Undyne the Undying is not the last super difficult fight in the No Mercy route.
Without getting into spoiler-ey specifics:
Do not use the Pie. Its MAX HP heal is too good to 'waste' on Undyne.
As for the Snowman Pieces:
If using the Snowman Pieces is what allows you to defeat Undyne the Undying, use them. It is better to progress than to veinly attempt to save every last bit of HP for later fights. With that said, it is better to avoid using them if you can.

Aside from those, what is the best 'loadout'?

Well, just fill up your inventory with the most powerful healing items you can get your hands on (listed above).
Obviously, leave the Pie (and Snowman Pieces if you're saving them) in the box.
Leave at most 1-2 slots open for Sea Tea, if you're gonna use it. As for whether or not you should?
It's up to personal preference, but I would recommend taking only 1: it helps, but two offers diminishing returns.
As for Dog Salad, the possibility of an extra bit of HP isn't worth the added RNG, IMO, but it's definitely not an invalid (or even sub-optimal) strategy, especially if you're gonna replace Crab Apples with it. The real issue is how tedious it is to get.

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Dodging Attacks

This is the part of the guide where you get a visual explanation. Here's all of Undyne's attacks done without getting hit! For these upcoming descriptions, "Arrows" refers to Green SOUL mode attacks, and "Spears" refers to Red SOUL mode attacks. The small arrows (↑→↓←↑→↓←) indicate which direction you should face for the attack (not the way the arrows face).

Attack 1: Simple Arrows 1


This attack is a variation of Undyne's normal starting attack in other routes of the game. It's extended to feature a few partial circle motions.
The pattern is:
↑ ↑ ↑ → ↓ ← ↑ ← ↓ → ↓ ← ↑

Attack 2: Simple Arrows 2


This is another simple attack. You only need to block left and right.
The pattern is:
⇐ → ⇐ ⇐ → → ⇐ → → ⇐ → ⇐ → ⇐

Attack 3: Random Arrows


This attack is random, but note that each arrow occupies a 'slot'.
Use this knowledge to give yourself an idea of when you'll need to switch directions.

Attack 4: Random Spears


This attack is random, and based entirely on experience and skill. And Luck.
My only real advice is to not be afraid to make some risky maneuvers.

Attack 5: Spear Columns


This attack is comprised of three columns. Spears will come out of the bottom, then shoot towards the top, one column at a time.
Stay in a single column until a spear appears below you, then move over to an empty column.
Note that the spears do not deal damage while disappearing.
This attack has a slightly difficult, but rewarding cheese strategy, shown below in attack 18.

Attack 6: Fast Arrows 1


This is another mostly simple arrow attack, but it tries to fake you out by suddenly sending faster arrows out from the left and right.
The pattern is:
← ↑ → ↓ ← → ← ↑ ↓

Attack 7: Fast Arrows 2


This is a super fast arrow attack. Multiple arrows will come from the top, followed by one from either the left or right.
The pattern is:
↑ ↑ ↑ ↑ ← ↑ ↑ ↑ ↑ → ↑ ↑ ↑ ← ↑ ↑ ↑ → ↑ ↑ ↑

Attack 8: Fast Arrows 3


This attack is comprised of a set of alternating arrows coming from the left and the top that suddenly switch to being yellow arrows.
The pattern is:
← ↑ ← ↑ ← ↑ ← ↑ → ↓ → ↓ → ↓

Attacks 9 & 10: Spinning Spear Circle 1


Yes, these two attacks are completely identical, and not random.
In this attack, spears appear in circles of six, and start spinning towards you.
I recommend finding a movement pattern that works, and then sticking with it.
The strategy that I use everytime is shown above.

Attack 11: Complex Arrows 1


This attack is comprised of arrows making circle motions, followed by a broken circle motion.
The first and last circles are comprised of yellow arrows.
The pattern is:
← ↓ → ↑ ↑ ← ↓ → ↑ → ← ↓

Attack 12: Complex Arrows 2


A mostly simple attack that requires you block left then right, followed by up or down.
The pattern is:
← → ↑ ↑ ← → ↓ ↓ ← → ↑ ↑ ← → ↓ ↓

Attack 13: Complex Arrows 3


Another mostly simple attack that requires you to block in one direction, followed by the opposite direction.
The pattern is:
← → ↓ ↑ → ← ↑ ↓ ← → ↓ ↑ → ←

Attack 14: Random Arrows


This is the last Green SOUL mode attack. It's basically the same as the last appearance of this attack.

Attack 15: Shooting Spear Circle


In this attack, spears appear in circles of six, then shoot towards you.
Note that the spears can't damage you after they converge, and how the spears aren't spinning.
My advice is to keep moving, finding gaps in the spear circles. Don't be afraid to hug the walls or cut it close.

Attack 16: Spinning Spear Circle 2


A harder version of the Spinning Spear Circle attack that has 8 spears rather than 6.
This is Undyne the Undying's hardest attack in my opinion. I barely managed to get footage of not getting hit by it.
A good strategy is to stay mostly still, and look for openings. Once you've found one, go for it, making sure to account for the spinning of the spears.
Do note that the spinning slows down as the spears get closer.

Attack 17: Random Spears


This attack is the same as the one from earlier, except much faster due to the progression of the fight. My advice is the same as last time.

Attack 18: Spear Columns


Like the last attack, this attack is a retread of earlier, but faster.
This time, I show this attack's cheese strategy. There is a pixel-perfect spot where none of the spears can hurt you. Make sure to hold the 'back' button ([X] by default), which will slow your movement, making it easier to get into position. I would recommend trying to dodge the attack normally if you're not used to dodging the attack this way; it's better to expend a bit more effort dodging instead of trying and failing to do a cheese strat.

From here on out, Undyne reuses her attacks in this order:

Simple Spear Circle; Spinning Spear Circle 2; Random Spears; Spear Columns; Repeat.

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Level (of Violence)

If you progress normally through the No Mercy route, your LV will usually about 10. If you go out of your way to kill enemies with high EXP rewards, you may get up to LV 12, or if you do the opposite, as low as LV 9.
What do these differences do?

Well, LV determines your base HP, AT, and DF. At LV 1, you have:

You may notice, however, that if you go into your STATs menu, it will look something like this:



What do these numbers mean? The numbers on the left are your base STATs (the numbers I gave in the above paragraph), while the numbers on the right are your equipment STATs. To find your "true" STATs, add them together, then add 10.
Why add 10? Because the STATs menu lies to you. No, really. It says your base AT and DF are 10 lower than they should be, for whatever reason.
Anyway, the actual base attack and defense for this example are 28 AT and 12 DF. Accounting for equipment STATs, the AT is 35, and the DF is 22.

Obviously, the higher your LV, the higher your HP, AT, and DF, and the easier the fight will be.

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Playing up to Undyne the Undying

If you're really struggling against Undyne the Undying, you might (keyword: MIGHT) want to reset entirely, in order to get the best items possible.

If you do want to get the biggest edge possible, reset, and play from the beginning of the game:

At this point, you should be as prepared as possible for Undyne the Undying. Note that if you want to change equipment, the last box you can find is the one in front of Gerson's Shop.

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Speedrunning

Let's say you've already beaten Undyne the Undying, and are looking to speedrun Undertale. Given that you're just starting out, you'll probably be running either the Neutral Glitchless or Genocide categories of the game. In that case, you'll not only have to fight Undyne the Undying, but you'll also have to get good at it.

As you may already be aware, if you're being optimal, you'll have the Ballet Shoes and Bandage while fighting Undyne the Undying, and you'll be at LV 10.

To get an optimal fight, you'll need to do 2 things:

  1. Kill Undyne the Undying in 14 attacks (you won't see the second Random Arrows attack)
    2. Seeing the Random Arrows attack is acceptable, but the Simple Spear Circle attack right after is very lengthy - try to avoid seeing this attack if at all possible.

  2. Not get hit while in the Green SOUL mode
    3. Internally, Undyne has a "Challenge" statistic. This statistic keeps track of how fast the arrows are in the Green SOUL mode. Getting hit lowers the Challenge level, slows the arrows down, and makes all of her Green SOUL mode attacks take longer.

Outside of the fight itself:

If you need some safety for the fight:

Oh, and one more thing:

If you use a healing item in battle, when the enemy finishes their attack, the curser will still be on the 'Item' option. If you're used to quickly pressing Z and Enter to attack, the you'll end up selecting and using the first item in your inventory. At best, you'll waste the Pie (and a turn), and at worst, you'll eat the Spider Donut in Neutral Glitchless. Do not eat the Spider Donut, as doing so will force you to kill Muffet and watch her ~20 second long death animation.

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Conclusion

And that concludes this overly long and comprehensive guide to defeating Undyne the Undying.
I hope this guide will help you with fighting Undyne the Undying, and I also hope that it wasn't too long or incomprehensible.
Thanks for reading (and bearing with me).

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