Fighting against 3 opponents with a little bit higher Score is much easier than against one with 3 times higher score.Characters with similar Score can deal high dmg to each other but if the enemy is to strong he OneShots you and you deal no DmG if you dont Combo him to oblivion. Use a fighter which can control the flow of the match. I use Resonant Evil BigBand with a lot of Stun skills.The opponent almost never act and if i **** up BigB can Tank quite good.
If you are lacking good units just skip the stage and level somewhere else.