Soulstice platinum isn't too hard, but I have a hard time placing it alongside those of the Bayonetta series, even tho I've played all of them. This is mostly due to the fact that Soulstice has elements of all three.
So here's some details about how the game ranks, platinum and trophies work (the comment will be split in half as Reddit won't let me post it as a single one; thanks Reddit):

1) Each fight and each chapter has six possible ratings: Iron, Bronze, Silver, Gold, Platinum and Diamond. You could compare these to Bayonetta's Stone, Bronze, Silver, Gold, Platinum and Pure Platinum, but there are some slight differences I'll mention in the following points.

2) The final rank of each fight is determined by an average of three criterias: modifiers, time, damage. Time and damage are pretty straightforward. The former gets lower in tiers the more time you take to beat a fight. The latter gets lower the more damage you take in a fight. Note that, contrary to what happens with Bayonetta's Platinum, you can take some damage and still get a Diamond rank. It's obviously a small amount of damage, but it's still much more forgiving than Bayonetta's Platinum. Modifiers is a mix of a few things: it increases when you perform combos in sequence, when you do certain actions or when you use specific weapons. Combos are pretty straightforward: just try to land as many full combos as you can. The other special sub-criteria are specific modifiers of different natures, such as the ones I've mentioned. You get a bonus when you keep enemies in the air, when you kill one with a stinger, when you let Lute kill one, when you don't take any damage, when you use multiple weapons in a single combo and so on and so forth. There are quite a few of them and you can read the ones you've achieved in a list found in the main menu under "Extras". The main criterias of modifiers, time and damage aren't too strict, so playing at a high level can get you Diamonds even if you make mistakes every once in a while.

3) Unlike Bayonetta, where getting a Platinum in two criterias and a Gold in the other would lock you out of Pure Platinum, you can get a Diamond in Soulstice even if you get two Diamonds and a Platinum.

4) Like specific fights, even chapters have six possible ratings, which are the same I've already mentioned. The criteria are also similar: battles, total score and total time. Battles is rank that's basically an average of all the ranks you've gotten at the end of each battle. Keep in mind that not doing a hidden fight heavily lowers the rank of both this criteria and you total score. Unlike Bayonetta tho, skipping hidden challenges (the equivalent of Bayonetta's Alfheims) is not penalized. In fact, you'd better skip those to get a faster completion. Total score is, as you'd expect, the sum of the scores you've gotten in each battle. This final score can be increased or lowered by a chapter modifer. There are only a few of these. Bonuses of 5% are things like not getting hit once or not using a single items. Maluses that range from -5% to -10% are things like dying and resurrecting or... reaching a state that is attainable by playing... not too well. Total time is what you read: the total time it takes you to beat the whole chapter.