The Golem is the penultimate (second to last) boss in the game, and is found in Brightstone, in the 'The Earthen Gate' stage. He is a melee boss.
Meteor Shower - A series of Meteors will fall on the field, doing damage in a small area around where they strike.
Knockback - He will knock his current target across the field, and then randomly select and attack a new target.
Fists of Fury - He will 'enrage' (like the barbarian's enrage ability) and will turn red. He deals much higher damage when under the effects of this ability, however moves significantly slower.
The Golem hits hard during the effects of Fists of Fury, meaning that a boost in healing/damage reduction may be required (depending on your character levels). For all other parts of the battle, he can be easily 'tanked' by any melee class, allowing your other units to focus on doing maximum damage to bring him down quickly. When he becomes enraged, a good idea is to do whatever you can to increase healing on the affected unit (using instant heals, damage shields, healing boosts...). Tanks are not a necessity for this fight, instead a Rogue with the +15% armor ability can be used effectively to kill the Golem faster. The Meteor Shower can be countered simply by using the Cleric's Group Heal ability, and the 'Knockback' is not a major issue, and is mostly an inconvinience, so long as you're paying attention.
Escorted Level Up Strategy (Info from RedGuard posted at Gamefaqs board under "Best way to level up?"):
This strategy requires:
Knight - 2x Codex of Jabe, Revenge skill (reflect 100% damage back when Shield Wall is activated)
Cleric - 2x Codex of Jabe, Guardian Angel skill (boost all healing)
Anybody else - 2x Mystery Egg
Just tank the boss with Knight with Cleric healing him as usual. When he Enrages, activate Revenge and Guardian Angel skills. His own attack does a sizable chunk of damage to himself. The time between Enrages is long enough to have both recharge in time, if not just kite him.
NOTE: A noteworthy strategy is to use a Paladin, when he enrages, use "Aura of Light" (lv5 skill) which changes damage taken to healing. Instead of taking mass amounts of enrage damage, you can be fully healed.