Time efficient 

early-mid game:

young bull warrior: With only 120 HP they are ez picking, no matter what typ of build you are playing. 200 exp is quite a lot. I use them to get to level 10 right at the start of the game.

weak Ghost: they only got 80 HP. IF you get enough INT to kill them reliyable  with a single holy fist, go for it. Otherwise they are pain in the a**. Bring some alcohol to refill your mana.

midgame:

Gargolye: You need the Jester Badge if you want them to fight as early as often as possible in the underground dungeon. Recommnded for Heros with high INT or CHA to ignore their high armor rating. Bring a lot of  alcohol to refill your mana.

endgame: 

Yeti: 550 Exp is the highest amount for farmable Enemies: However he got a Grap/Hold-attack. With very high STR, it's no big deal to struggle free. If your STR is low you should avoid farming him. Another option is to ignore his Grap Attack and keep FLIRTing. This is only adviced with very high CHA Stat.