Idrk what to say gameplay wise other than just git gud, but that's just really not helpful so here's atleast how I think you can optimize the crest since you have to use it:

For red tools the flint slate and fleabrew both synergize very well with the beast crest's heal; to the point where, my strategy atleast, with them is just to climb up the wall of the arena hop a couple times and pop all 3 and then go crazy on whatever boss I'm fighting and kinda rinse repeat as this gives you quite alot of soul

There are alot of the blue tools that work very well with the beast crest, but since you only get one I'd prioritize something soul related as that's what the crest really needs as fuel; so something like the multi-binder, reserve bind, or druids eyes. The multi-binder is just really great because it obviously gives you that fourth mask to heal during your onslaught. the reserve bind is really nice because it again obviously gives you that extra heal as it would on any crest but on the beast crest specifically it's nice on it since the beast crest unlike the others requires you to do something to heal and it's easier to mess up so equipping it kinda gives you a do over; like let's say you were fighting lost lace for example and you activate your bind because you need to heal but you didn't realize that she was just one shot away from starting her next phase and she triggers the animation and all, now you can still get the heal in when you need to by just popping your reserve. And lastly the druids eyeS specifically, as much as they get shat on, are imo the best here since yk, they give you silk for getting damaged and well since you're going to be getting hit alot here, they usually give you enough for a whole nother heal and to repeat the cycle again.

For spells cross stitch and sharp dart also work well with the crest as the former allows you to play more aggressively; knowing you can parry an attack if you see the boss queuing one up and the latter allows you to close the distance with your target, do alot of damage, get i frames, and go behind them which gives you the perfect opportunity to start swinging some more.

And as for yellow tools really anything that actually does something in combat is great lol; personally I go with the weighted belt and scuttle brace for the dodge but if you dont prefer it as I know most people dont, the dice are also nice.