imo, Grimm, (well at least his Nightmare fight) was the hardest boss in the game -before Godmaster dlc decided to ramp things up
there is absolutely nothing wrong with you getting all other stuff cleared out the way or collected before you go at him,
-while he does have a fairly reliable move-set, it’s one of the harder to get adjusted to and react well and in time to, full upgrades can easily be recommended
regarding GrimmChild charm, until it’s fully upgraded (which will require you to beat Grimm twice) it’s not worth a slot except for very weird charm combos where you attempt to kill a boss with “adds” mainly -which all deal low dmg btw
i think, don’t quote me, grimmchild is like 4dmg after first “quest”, 8 after beating Grimm, and 12 after Nightmare. So unless you get grimmchild early game, it’s a waste of 2 slots for only that low dmg, and you can easily replace with more efficient. Also, low rate of fire, and just… doesn’t really seem to target optimal on stuff with adds it feels sometimes.
Colosseum of Fools - highly recommend you attempt your meddle in the arena
the way i did it was clear a ton of the game, beat Grimm, clear whatever i hadn’t found, and do Nightmare Grimm at the end(like xist for about 2+ hours lol). And then totally breezed through the game’s final boss(es) like it was no big deal, -coming straight from that prior ordeal
oh, and (imo), if you think regular Grimm is tough, you’re gonna love a couple of endgame bosses in Godmaster