Skul: The Hero Slayer

I got this achievement legit. I don't think its possible to get a no-hit on this boss (or any boss after) without completely upgrading the witch's buffs and RNG being in your favor with good items/skulls and being able to kill them quickly. If you are average skill, these are the things you need to know going for the no-hit achievement with this boss specifically:

INVINCIBILITY FRAMES

Dashing in Skul is sort of like dodging Dark Souls: if you do it right, you'll go through enemy attacks without taking any damage. This is actually key to being able to beat the bosses.

INFINITE TRIES:

Obviously, the hardest part of this achievement is not getting hit and you don't want to restart the entire game just for another try. Thankfully you can save scum and retry your no-hit attempt on this and any other boss as many times as you want by doing the following:

push
hilight "Skul: The Hero Slayer"
press
hilight 'quit'
press

When restarting Skul, your save will put you back in the previous room and you can retry the boss.

'OVERWHELMING POWER:

The easiest way to get any of the no-hit boss achievements is by defeating them as quickly as possible.

Being that the Leiana sisters are the 2nd boss of the game, its hard to be lucky enough to have the right combination of skulls/items/quintessence needed to demolish their health bars. It -IS- possible though.

The 1st half of the Leiana sister fight is also easy enough (its still hard!) to defeat one of them without getting hit. Play it enough times (and if completing Skul legit, you will), and anticipating their attacks will become second nature.

The 2nd half features a few attacks that are very tricky to avoid. If you can't absolutely crush the 2nd sister in seconds, you must know how to dash through the bigger attacks to not get hit during this part.

Here's what I had when I got this achievement:

I had all the witch's power ups completely purchased.

Quintessence:

Shadow Knights (would not have been able to do this achievement without it)

note: magic damage equivalent quintessence is Archdemon.

Skull: Petty Thief (with 'throw gold')
(I'd say Alchemist skull is also pretty good so you can hold attack down and focus on purely avoiding attacks)

Items: anything with + physical attack

note: I had execution axe, master fighter, and 2 items with 'courage'. total extra phys atk = +120% which, honestly, is weak and I could have done better. I did have to retry for the no-hit several times with this set up. Importantly: it worked.

I'm not a master at this game. I'd say I have average skill.

battle part 1:

Once the battle began, I immediately used Shadow Knights. A single use will usually knock at least 1 sister down to 1/4 health or below. I was also experienced enough to finish one of them off without getting hit more than not.

Its kind of important to use Shadow Knights right away despite not getting the most damage out of it. Mostly so by the time the 1st sister is defeated, it'll be close or fully recharged for the 2nd part.

battle part 2:

F**K THIS PART

I mean

This part is tough and way harder to avoid a hit. The boss is fast and doesn't give melee skulls many opportunities to attack. Its why I needed a projectile attack like throw gold. If you get too greedy trying to get melee attacks on the 2nd phase, you will get hit.

There is at least 1 difficult to avoid screen filling attack the boss does. The screen will fill up with slash lines telegraphing damage zones when the attack is executed. If you are not lucky enough to have a giant open spot to stand, don't panic! You can avoid damage if you time a dash just right.

When I got the no-hit achievement, I was keeping my distance and using throw gold (read as: projectile attacks only). I didn't have shadow knights recharged when the 2nd phase began and used it as soon as I was able. And managed to pull off a no-hit victory this way.

NOTES:

'no hit' means 'no hit' and not 'no damage'. I'm saying this since the first time I defeated this boss without taking damage, I had a bubble shield, got hit, and did no get the achievement.

Criticalmanc in the comments claims the barrier shield does not invalidate the achievement if it gets hit.

Skul is also horrifically unbalanced, and the RNG is terrible. If you're having too much of a hard time getting a no-hit win on this (or any other) boss, don't be afraid to press on. You may have gotten unlucky with your character build and you might wind up with a much stronger build the next time around. I have spent hours pulling my hair out trying for no-hit achievements on Skul only to have a much easier time on a later game because I wound up with a better build.