Update! My group had a summon capture tonight, and I did defeat one of the hostile elite ninja squads with a team (I didn't even notice that there were three before; there's an Ay team, an Onoki team and a Mei team), and this time I noticed something dropped for defeating them. 1 Spirit (which is one of the 3 buffs you can get during the first phase), 50k coins and 5 coupons. It was... quite underwhelming, haha. But it's something, I guess? It also looks like the damage you do to the hostile ninjas counts towards your total damage. I was late to the summon capture, missed the first phase and didn't even reach the snake to fight it (since I got ambushed lol). so all I did was the elite hostile ninja fight, but I still got 15 coupons, exp, coins and a summon frag for it.

Also, note that in your team, whoever is in the center is mostly likely not gonna get to do much against the elite hostile teams. The middle team will be soaking up the damage and debuffs, mostly. xD