Level 95 Hau, Level 95 Nanu, and Level 100 Cheren have almost no problems auto farming Hard Co-Op...I just don't plan to try for Very Hard since they are more likely to need more specialized units that can be obtained from scouting. Just in case, I try to not have NPCs, although I managed fine with just one other player. You can complete all the quests for the event just by continuing to play, 100 battles against Giovanni, Single or Co-Op, will complete all quests. The rest is farming vouchers to not only unlock Mewtwo, but make him much stronger.
EDIT: I now have Giovanni and Mewtwo unlocked. I'm continuing to do the farming since it is possible to get the stars increased on Mewtwo. I may as well see if I can power-up Mewtwo a lot so it can be a very good striker alongside Raichu.