Who should I level up for Prisma event

If this is just your female servants then you're good. Just raise Medusa a bit and Medea for your arts damage dealer, but it's pretty good. Heard the event wasn't all that hard, but just had lots of missions, and other than that just ask around for someone who has a Mebd or drake, a good lancer and an arts damage dealer other than that that's it.

Ushi, Medusa and St. George are all you need for bosses. Mozart, Medea and Hans are good support. Honestly I think you're set.

I believe in the current roster, Ushi is actually the best single target NP female rider. Her only real drawback is that since its a quick NP, she doesn't benefit from any of the event CEs.

Focus on Medea, put her on lv 60, as her 3 Arts deck and Rule Breaker will greatly benefit from Kaleido Sapphire (+50% arts vs the raid Bosses).

Ascend her if you believe you have time to do her 3rd skill Quest, but dont evolve her further than 60, from 60 to 70 she gain very few HP and attack (less than 900 I think), so it will be a waste of embers at this point.

Level Medusa some more (max +7 if you can, or at least +3), even without lolCEbonus, she will be a good farming machine against the nodes who have many casters, because of AOE+Nobstrous Strengh Skill. (level that as well, if you have mats for that).

Kiyohime will be you main Buster (Kaleido Ruby CE holder, +50% CE), but she's already maxed...Shakespeare and Leonidas can help her damage with their Buster Ups, but put a starting NP CE on her, when not in a raid boss fight.

Of the Males, Hans skills will keep being useful, as allways (it's not like his cards are even a good damage option, this event or outside of it), so if you have some caster gems, level them a bit.

Georgius 3rd Skill is Battle Continuation, while it combines well with his tanky kit, you better of priorizing Medusa over him.