Basic Trope: Distinct, powerful enough enemy that awaits you before the confrontation versus the actual boss of the stage.
Straight: In the platformer Super Troperia Adventure, each stage has powerful enemy in the middle of it, placed well before the actual boss battle for that stage. Unlike other enemies, they can't be skipped so you know that they are (mid-)bosses.
There are a dozen or more of those kind of enemies per stage.
Fighting the mid-bosses makes for more than 2/3 of Super Troperia Adventure.
Downplayed: There's one enemy which is set apart from the rest by being much stronger than any other enemy besides the boss, but it can be skipped just as easily as the others and generally isn't any more dramatic.