Your best bet is a place with many purple encounters that you can easily clear.
The underwater portion at the very bottom of the first dungeon should have a purple enemy (big fish thing) patrolling around each "pillar" near the stairs so it should be a decent area if you are strong enough.

Not sure if there are other spots with purple enemies near stairs.
In theory, the super enemies when you are over mana can count but they tend to be rather dangerous.

Of course, normal fights still give you some pact exp but purple fights give so much more that it can be worth finding a good spot for it.

If you already approaching the end of the game, go to level 1 of first dungeon bring mana over the limit to farm death gods. Death gods (the super enemies as the previous comment said) in level 1 have health around only 20k(but they do have high resistance of all attacks), bring supporting puppets as substitutes to avoid instant kill.