Leech ISN'T fine, because such a huge percentage of the enemies in the game rely on physical damage.
Even if their AI was smarter, what are they going to do? Respec? They only have certain tools to use. Unless you want to the game to analyze the player's build and start changing enemy stats and abilities to counter the player...
The player was never intended to become so resistant to physical damage, without investing HEAVILY in defensive skills and equipment, like armor and shields. You can put leech on a scoundrel, and they instantly become as tanky as if you had equipped them in plate armor and a shield. That's not balanced.
Use Buffs, Debuffs and Board Control. There is plenty of stuff to spend your AP besides hitting a immune target. There is most of the time other targets to at least try too. If the NPC is supposed to be a thread to players than doing them on one damage source alone is a problem in itself.
