BioWare hit the sweet spot with mid range boss fights in Balders Gate 2 imo. You often faced groups of higher level enemies of differing classes which presented a good tactical challenge without being cheesy. In later games these type of groups are not as well rounded and easier to beat. (One good mini boss fight in ME was the krogan and his group after you rescue liara. (Terrible that they had an unavoidable cutscene right before that battle mind you.)
So a well rounded enemy team will have some heavy hitters, some biotics and some techs.
One thing that BioWare has totally failed at over the last number of games is big boss fights. They can't seem to balance them out and toss out the same formula. Big boss fights a bit, gets hurt, flys away where you can't hurt them, a bunch of minions come out, rinse and repeat.