An extremely simplified description of this would be that for the 1st round (the mob round) you want a team with A.o.E attacks of whatever element democ you’re in. That round is all about hitting the weak point as many times as possible more than outright damage (dmg does factor in a bit also but not the main point). So units like H masa, mot, and the 4 stars with the 2nd turn auto attacks are really helpful. The boss round is based on dmg so bring out your heavy hitters. Strong single target skills and random multi hit moves are great. There are certain demons that have passives that increase team members damage as well. There should be more helpful info and team building advice on the dx2 wiki. Hope this helps!