Will Santa Alter be good enough against caster bosses for the event even though she's aoe?
Just see if the boss is weak to buster, equip Salter with the CE and you're set or if you're not sure go borrow someone's thot (Mebd). Honestly I'd just go around with my Drake, so it's fine since we have the CE, bosses are weak to either arts or buster and stuff.
I have rider mordred and medb just not leveled, so if i were to choose one to level which one should it be?
Mordred if I have to say. I mean Mebd works on her own with some problem with her NP gain, but could just ask for NP charge like partake with the king (which she couldn't equip for the event), but for Mordred to really work you need Tamamo so...raise mordred and when you're fighting a caster boss weak to arts borrow Tamamo and when buster is the weakness borrow Mebd, but honestly just level Medb for a bit during the event. I recommend at least level 60-70.
Ushi will only help since she's quick. Go find Mebd, but honestly Salter's fine since she's at least buster which gives bonuses (50% increase) add that to attack power up and you're fine.
