Soulstice

This achievement requires getting diamond on Mission 24 and 25. I used the the Trickster Rapture state to get it. It seemed to provide the most amount of damage to a single enemy when compared to the other rapture states.

For Mission 24, stay close and time your combos and dodges. Don’t use long combos because you’ll be locked in an attack animation, unable to dodge. If you get to far away the boss will spawn either blue or red orbs that’ll track you. It’s easier to just stay close and dodge. Only use long combos when he’s down. Obviously try not to get hit. It is possible to get diamond if you get hit once or twice. Anymore and you won’t have enough time to increase your overall combo multiplier for your score. From my experience this was the easier of the two missions to diamond.

For Mission 25, it’s a lot harder to get hits in for your combo score and rapture state build up. For that reason you can’t get hit. As far as I can tell, if you get hit once you’re score won’t recover and your time will also be longer, resulting in a platinum.

Not a guide as if you're at this point you shouldn't have much trouble with this. Just a clarification that this requires you to Diamond both Chapter 24 & 25.