I think it will be Colosseum next.

If I'm correct, the winning strategy is to just use the highest SED in any one girl type. Have three of one type and two of another type, and get rid of your weak friends to make sure your backup is strong. (Or simply mix everything and not have a type, but then nobody will attack you).

Assuming each girl you play has lv 100 SED, it does not matter what you choose, as it will automatically lose to any one other girl type. The key is to make sure you are beating girls of the SAME type.

For each category, the top three lv100 SED girls are:
MOE: Hiyatora Katagiri(11,000) [in Gatcha], Kanae(11,100) [prize for 150 Super Elite Guards in first Elite Guard event] and Naomi Hashimoto(10,990) [???].
BABE: Miu Minazuki(12,020) [prize for Rank in 2nd PeroPero gambit event], Mako Fujioka(12,000) [gotten from chance in stage 2.1], and Michika Tanabe(11,950) [???].
SEXY: Lilith Lil (13,120) [Facebook event], Shiotsu Rino (13,070) [???], Yusa Yagami(13,060) [prize for rank in Elite Guard 4].

Focus your EX wild cards on these, if you have them.