JCoxeye's Guide to Fighting Undyne the Undying

This will be a comprehensive guide to defeating Undyne the Undying, including strategies relating to leveling, equipment, using consumables, and dodging attacks.


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, have started a Genocide/No Mercy route, and are looking for a comprehensive explanation/strategy guide for the fight with Undyne the Undying.

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.


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

Note that once you fall into the dump, backtracking to get the Ballet Shoes and Old Tutu becomes impossible.

As you can see, the Ballet Shoes and Old Tutu provide better ATK and DEF, but the Torn Notebook and Glasses provide extra INV.
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:


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.


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'll die after taking ~5.33 hits).


Your INV STAT 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 thirty frames per second, 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.

Based on their descriptions, Torn Notebook:
'* "Torn Notebook" - Weapon AT 2
* Contains illegible scrawls.
* Increases INV by 6.'

Cloudy Glasses:
'* "Cloudy Glasses" - Armor DF 6
* Glasses marred with wear.
* Increases INV by 9.'
'* (After you get hit by an attack, you stay invulnerable for longer)'
one might assume that the Torn Notebook and Glasses would give 6 and 9 INV respectively, but that's actually not the case.
In reality, they give 15 and 30 INV respectively - a very potent buff that will save your, uh, grass.

Wearing both will give you 45 INV, which will over triple your INV STAT. The Glasses alone will still give you over twice the INV, so wearing them is still more than worth it even without the Torn Notebook.

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 Glasses add more INV anyway, if you're gonna use only one INV boosting item, it should probably be the Glasses.

As for the three remaining sets (Shoes + Tutu, Shoes + Glasses, Notebook + Glasses):


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:

Items you can backtrack to get:

Items you can't backtrack to get:

Which of these are the most useful, you ask?

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 vainly attempt to save every last bit of HP for later fights.
With that said, it's 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 one: 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...

Dodging Attacks

Each of Undyne's attacks, and how to dodge them.

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).

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

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

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.

So, 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:

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, making it seem like you start at 0 AT and 0 DF.
Why it does this, I do not know.

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.

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 really 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.


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)!