From memory (might be hazy):
There's a single room, I believe somewhere near the end where you find paul (one of those two), where there are only one or two enemy models.
They respawn each time you reenter the room if I recall correctly, and occasionally they'll have Leech Kings.
I myself encountered my first Leech King in a Katz Mission (the Dwarves Ore Mines) and it slaughtered my party. These things are a lot higher in terms of levels compared to your party when you first encounter them.