I think the closest thing to the DLC in the main game, waterfowl dance aside, are some of Morgott and especially Maliketh's second phase moves. But chances are you'll get the phase transition when they are really down to 40% or less HP, and they aren't that tanky to begin with, so yeah.
(Another thing to consider in these games is AI RNG. 10% of your attempts will also have 10% of the most beneficial AI moveset being used against you, so have you really powered through the boss, or where you just lucky in a boss that takes that many attempts?)
Also, story-wise for the DLC: