I don't think that decision is up to us, but no harm in speculating.
I'm thinking Red vs Browbeat. Despite the memes, his true power of skin-level telekinesis and biokensis makes him a tank. He might be able to 1 v 1 the giant Dragon-Bat with no problems. If he doesn't fight Red than Thorne might be a decent fight, but that depends if he can pull an adequate hit and run campaign. Thorn's 1000 needles may or may not get through Browbeats' stone flesh armor.
Next, maybe Buoy vs Vista? With all the spatial warping, most attacks are a no go, so to prevent a clean sweep something more esoteric is required. Buoy sort of has that with strength sap and absorb. The issue than becomes Vista smacking down a frilish like she's a biotic, because physical attacks don't work on frilish.
White, if we have to employ him before any of the new summons get here, will fight Gallant. Mostly to work Gallant's emotion powers to stop a physical threat.
I guess, the challenge is giving Shadowstalker someone adequate. I don't want to have Scalpa do it as that'll be a bad experience for both; Shadowstalker needs a shower, and Scalpa deals with her attitude. Can't be a pure animal either so White and Red can't do it. Zap is technically her weakness( electric for the win), but again, it feels insulting to both. Moreover, I can imagine Zap getting punted, so Sophie neither works for her victory nor does it feel earned. Alyamon could try, but I feel like this could backfire.