Little strategy I found to train a unit up at lightning speed:

play as altar or a custom race with henchmen and quest maps.

make 8 bards, and give them all the governor class. 

add the unit you want to train into the stack. 

Send your army of bards on quests, and they will train pretty quickly, but put all your levels into the commander side of the skill tree so that you unlock the trainer skills. Give all of your bards the trainer skill until it is maxed out. For added fun give them all the command skill To be able to keep using one powerful unit repeatedly in battle.

henchman don't split the XP so every unit in the group will be getting around 300 XP from even small groups of spiders.

keep questing and your units will go up a level or 2 on every quest