Mythics will count as max merges depending on the season, so you're encouraged to have at least one of each type.
I managed a 750 score with my +10 Arena units, a +2 Peony, and a +1 Eir. My Tanya, LLeif, and Igrene were paired with those Arena units to keep the high score, as they went about the map Galeforcing in Leif's case, and AoE-spamming in the others' case. This is probably a valid strat to go with as you go up the tiers, so you're not blindsided by Stance users and enemies with super high HP that can counter back.