Which servants should I focus on?

Based on your preference. I see people just making them reach max ascensions and slowly levelling them up since skills make the servant, and some people who like me want max leveled servants. You're pretty much set for the event with high level female servants (level 60 in the least), and only needs to borrow from friends so I suggest going for nursery rhyme since she's going to have added damage to the event (this is inherent to her and not through CEs) then Lancelot. If you do find difficulty then raise your levels to max and see, but other than that...I have nothing more to say.

Level up Cu lancer to get servant that can tank for quite a while. Hans you should level up both level and skills as he will be your main buff support unless you are using Support Waver. Scathach i am guessing is assassin is a good choice to also level up seeing as you lack a good assassin at this point.