I don't think annoying enemies and small health bars are the way to go. He should be trying to inspire the players having a fun time dispatching enemies.
If enemies are "annoying" (threatening) and if there's a significant amount of risk posed in dispatching them, then there'll be tension and a desire to dispatch them, and the player will have fun trying to defeat them. Item grinding/killing enemies to get gold is not fun, it's boring. The problem with leveling systems in action games is that you're often either too weak and fighting regular enemies becomes tedious and drawn-out, or too strong and the game becomes boring since you can just bulldoze through enemies without needing to care about getting hit. The player shouldn't have to be the one who balances the game.
I meant annoying as in cheap. Being in a room filled with hard to avoid enemies that can kill you in a hit or two is annoying/cheap.
Once I think about it, he should be focusing on making them fun to kill. Also, powerups is a good way to go.