Which servants are good for helping support Cu Alter

From Waver and Tamamo,Waver.Shakespeare buster buff is pretty useful so yeah you should level it higher and if you want to get him "broken" Start saving for Merlin my friend

Mashu + Andersen is enough. He is hard to kill when alone, with them he is literally immortal.
When Leonidas' third skill is out, you can use him too for more offensive setup.
The thing with Cu Alter is, you don't need high tier support (or support at all, tbh) to make him 'broken'. Just 6/10/10 him, he is pretty much can solo the last boss already

Cu Alter is very self sufficient so he doesn't really need top-tier supports.

Leonidas is a good option since he has a taunt and a buster buff or Mash for the defensive buffs and 0-cost.

Since everyone is asking the same question, I will just cnp my answer.

Mash and Nightingale. They can keep Cu Alter alive with heal and DEF buffs. Nightingale also have 50% Buster Up for 3 turns, with only 5 turns cooldown at skill LV10. They also have 2 Buster cards each, so they can help complete Buster chains to maximise damage output.

And since you mentioned Merlin, switch Nightingale for Merlin when he is released. Mash will be more reliable support later on.