I've got one I realized I could do tonight. Mikage + Yumeko. For Hydra I keep Mikage in her Base form, she and Yumeko both have Relentless BUT here's the key thing:

Mikage's A1 always has Yumeko team up (only other Shadowkin) so every time Mikage A1s, Yumeko has that chance to reduce her CD. Plus Deterrance Mastery on both (shame when Mikage counterattacks it doesn't trigger the Teamup) with Fear head proc'ing it. Plus Relentless on both triggering Extra Turns (and almost every extra turn for Mikage is another chance for Merciless to proc on them both with A1 and A3 Teamups).

I think it could get pretty impressive. You'd have to manual it but people are talking Trunda + Yumeko, well hell throw Trunda in that mix and it might get insane! Anyone else thought of combos like this? I'm also thinking Martyr or any other AOE Counterattack buffer