Who should I save up for?

Hi!

Merlin, of course, is totally broken, but with Tamamo and Waver, it seems you have your bases covered. Gilgamesh is also always good--like, really good--and extremely handy to have, especially if your only AOE Archer is Emiya. In my opinion, those are the two to gun for. However, all of the ones you've mentioned are good. Since it seems like your only leveled sabers are AOE, Musashi may be useful.

Sorry, this is such a nonanswer! I think Gilgamesh has a rateup around the post-Halloween time, and Merlin's comes with the singularity after Babylon. Shimosa is later, so if you want Musashi, you can roll for the others and save up a little more for her. Personally, I'm saving for Caster Gilgamesh.