What miniboss (and lv) are you talking about exactly? Because i don't see a problem with those so far, aside from the aurora knight's regen... (i'm only lv69 and doing lv95~ expeditions for the moment tho, so maybe it gets tougher, just wondering what boss got ya!)

I have heard it doesn't scale well at the higher exp levels. I'm only at 112 though, so I can't say for sure.
The damage does seem a little spiky for my liking, I could go higher if it wasn't.

Since your gear is capped at level 60 there should be an expectation of a rather flat difficulty curve past level 70-ish, as only gems continue to improve drastically at that point. Only times I find things difficult is when I play with others and when bosses cheat by having broken skills or by trapping me outside of playable space, which by the way... you can not escape with teleport skills for some reason. Who had the bright idea of making things so much harder when in a party compared to solo, I do not know, but it will not do wonders for the longevity of the game.