Downplayed: The levels are challenging, and the boss would be just as challenging but it uses the exact same gimmick as the levels, so players don't have to change anything to beat the boss if they got through the level.
The boss is a weak but brilliant and charismatic Big Bad who is protected by his legion of Mooks and deviously constructed Death Course.
The game is based around navigation and platforming, and not around combat. The bosses are simply there to signify the end of a level.
Alternatively, the game is a Bullet Hell shooter. It's significantly easier to avoid intricate bullet patterns from one enemy rather than sprays of bullets from several.
Inverted: Type 1 is an inversion of Type 2, and vice-versa
Conversed: "Oh man, after that level, the boss music has never sounded so sweet".
Deconstructed: The boss has an inferiority complex that leads to suicidal tendencies. When you fight him, he refuses to fight back and begs you to finish him. When the player beats him (if the player beats him), it's a genuine Tear Jerker.
Reconstructed: ...But when he finds out that you're actually going to go through with it, he has a Freak Out and tries to take you out with all he's got!...only to be taken out in half a second.
Type 2 - Easy Levels, Hard Bosses
Straight: The bosses are much harder than the levels.
Downplayed: The levels are simple and enjoyable, if only a bit. The boss, who does things that the level doesn't, is more difficult because the player has to learn how to cope with the different pace the boss sets.
Zig Zagged: There are lots of bosses in the game. Some are hard, some are easy, some are all but impossible, some are too easy....
Averted: The bosses are as challenging as the levels themselves.
Enforced: "Sure, the levels look pretty, but it's the bosses that'll stick in the players mind. So make the levels easy so they can get to the super-tough bosses, where the real fun lies."
Lampshaded: "If you're such a Bad Ass, why did you hire such wimps to guard you?"
Invoked: "I can't trust my Mooks to take care of this guy. It's time to take matters into my own hands."
Defied: A very tough boss battle is fought...for a few seconds or turns. The boss then escapes so he can stay alive and see his Evil Plan come to fruition.
Conversed: "Oh crap. That's the boss music!"
Deconstructed: The boss's minions are on strike and do nothing to stop you on your way to the boss. While you're busy with him, they terrorize the countryside, and one of them becomes the next Big Bad.
Reconstructed: ...Only for the first Big Bad to beat the arising boss and take control of his gang (and former minions).
Y'know, after fighting all the way through Boss Dissonance to get here, this page really isn't that tough.