Cleric

The Cleric is consistently the fastest end game farmer, as opposed to the Ninja who can be faster but with more luck involved. The Cleric is also easier to level since only Heal and Holy Strike (along with base level HP and DMG) are required to consistently farm.

While Retaliation is not needed to ever increase in level, Resurrection can be useful at 80-90% initial HP.

The biggest key in consistent farming is getting Holy Strike near 80% chance which also increases the DMG (a secondary consideration) and keeping the base level HP (along with Heal) to be able to take hits until Holy Strike obliterates the enemy. This has a secondary effect of raising the base level DMG (along with Holy Strike) to be able to destroy most non-Boss monsters with one hit (when Holy Strike multiplies the base DMG). Resurrection is used as a back-up until enough Health Potions are found.

Heal
Heal is the only Cool-down ability of the Cleric. Unlike other Cool-down abilities, Heal happens automatically every 15 actions.

Heal takes effect before any combat round. For instance, if Heal is about to happen (one action left) and the next action is a combat round that would normally kill you (the result ends in taking DMG greater than your HP), Heal will kick in before the combat round. This gives Heal a somewhat limited Revive type ability, circumventing the Resurrection ability or death.

Heal also takes place after a full Health urn, giving full HP before Heal kicks in. (More data needed to confirm)