Of all the boss fights that you need to complete with "No Damage", Mistral is the easiest.WHAT YOU WILL NEED FOR THIS TROPHY:
1. A backup save (just in case
2. Basic understanding of the parry system
3. PatienceWHAT WILL MAKE THIS TROPHY EASIER TO OBTAIN (but not necessary):
1. A maxed Muramasa blade
2. A maxed Sai (this will stun her, but it can be challenging to get the damn thing to hit her with all the other dwarf gekkos running around
3. Basic understanding of the "Offensive Defensive" maneuver (direction + Square + X)WHAT WILL MAKE THIS TROPHY NEARLY IMPOSSIBLE TO NOT OBTAIN (in conjunction with the items above):
1. Infinite Wig B (for unlimited Ripper Mode)
2. Custom Red Armor (increased damage)
The reason why this fight is the easiest is because it is broken up into 3 parts, with each part initiating a checkpoint save. This breaks the fight up very easily, and allows you to restart from said checkpoints if you take damage during any of the fights 3 stages.
For example, if you get through the first stage with no problem, but during the second stage you get hit, press the start button, and choose the "Restart" option - you will be back during the second stage of the fight with that nasty hit being wiped from the games memory.
Please note you need to be on at least Hard Difficulty for this No Damage boss fight to mean anything at all.
During the first stage of the fight I basically just used the Ninja Run Cut-Em-Up paradigm, immediately initiating Ripper Mode, circling Mistral to deal her damage, but also taking out those annoying dwarf gekkos. When the option comes up to use Blade Mode I didn't take it - I just kept cutting her up. With Custom Red, a maxed Muramasa and in Ripper mode you will get her to 70% health literally in seconds.
Then a cut scene will be initiated where Mistral hog-ties your leg. Unlike the previous difficulties, you need to cut a specific part of the pole arm in order to get through this, so be warned.
After the cut scene you will end up on a pipe. Initiate Ripper Mode again, and run towards her - she will take higher ground and start throwing those dwarf gekko bodies at you. You can either parry OR stand underneath her - your call.
She will jump down, and you can start cutting her up again. Here is where the parry move becomes critical: unlike the first stage, you are very limited with your movements. Parrying her attacks is a necessity. If you are having difficulty with the parry move, I suggest playing on Normal and getting to the Monsoon fight - that guy will make you a parry savant
Get Mistral down to ~50%, and another cutscene will take place, as well as another checkpoint save. This is basically a longer version of the first stage of the fight. I went with Ninja Run Cut-Em-Up again, while in Ripper Mode, however be careful: she is standing next to a few explosive barrels that, if you cut, will knock you back a few feet AND take away your "No Damage" bonus (thus negating the trophy). If that happens, restart from the checkpoint.
This is where I found the Sai helpful - it will stun her, eliminate the distance between her and you AND do damage. Wins all around. Unfortunately you NEED to take care of most of the dwarfs first, or you may target them instead, and leave yourself wide open
The Pincer Blades are also devastating to her, however I rarely had a moment to effectively use them, as those damn dwarfs would jump on me leaving me vulnerable OR hit me. Use those with caution.
Also, this is where the dodge (Offensive Defensive) move really comes in handy. If you see her glowing gold (unblockable moves) try to get in close and use the dodge - it will give you a second of invulnerability AND deal her a decent amount of damage.
Once she gets to ~10%, a QTE will occur. Initiate that, and you are in the clear as long as you took no damage. If you see that you have, restart from the checkpoint and try again.
If for some reason you botched up and didn't notice you took damage in one section, go to your backup save. You may have to start from the beginning of the battle, but at least you don't have to run through the level again.
Questions, corrections and neg vote justifications below.