How do I defeat the 3rd boss?

Store up health potions in the levels above. Start with throwing anything at the boss that will set the boss on fire. Then stand next to the boss and repeatedly wack it with your weapon. If you have anything that can poison the boss, use that too. Judge the average amount of damage the boss deals and when your health gets within the danger zone, drink a health potion. Every time the fire goes out on the boss, toss another thing at the boss that will set it on fire again. Don't run from the boss as it heals itself every time it moves. Eventually, because it is taking damage both from fire and from your melee weapon the boss will die. Hopefully it will die before you run out of health potions. Once it is dead, step away and wait for the poison cloud to disperse before going back to where it died and picking up the key it dropped.