Who should have limit breaked CE in my support?

IMO, Probably Cu, he's just a good tool to solve all tough problems in the game :P

Edit: Sorry missed second question first time. As far as placement for Cu, I would prob swap Raikou and Cu to make him easier for others to use since I believe second waves will have just like mainly one big bad that needs to be cleared fast and he's pretty good for that.

Just change Cu when the challenge quest arrives, raikou haves a good job in that slot atm and when said challenge quest is here then slap to cu that Limit broken CE so that nobody escapes from his monstrous ST np.
Edit: also try experimenting about changing those 2 in their places and let the friend points speak

FYI, the CE to have limit broken would have been the Fondant, not Sweet Crystal, as an MLB Fondant gets you an extra 5% divine damage, while the MLB of SC just gets you the extra NP charge (which Fondant also gets).

As for placement, personally for everything until maybe the Royal difficulty, I've been running with AoE servants who are strong against whichever class I pick with a mold. However, at Royal difficulty, I will probably run with a ST and an AoE servant, but probably won't run berserkers at all since it makes defense almost trivial. Just my support caster typically gets neutral damage (I haven't been picking Riders, just Saber (or Lancer)/Caster since those don't have short charge bars, and Casters don't do as much damage as well. With that lineup, all the way through Sweet, I clear the 2nd and 3rd waves in one each. I haven't done Royal enough to have optimized it, but I still don't see me running with Berserkers.