Dungeons 3

This is done in the "The Hungry, Hungry Ogre" level. Pretty straightforward, just keep the Ogre's health above 75%. In this level you'll find an Ogre that fights for you as long as you provide it with beer. Much like the Crowley level, you cannot control the Ogre and it will take on fights on its own once you feed it beer (taken automatically from your dungeon). As long as you push your forces ahead of the lumbering Ogre and pay attention to patrolling Heroes who may look for it while it's napping (Demon Gate spell comes in handy here), you'll do fine. The Ogre is also quite tough and will be a strong ally to your forces. Healing Naga and ranged attack creatures are recommended.

Pay particular attention to when the Ogre has defeated the Treant and falls asleep: an attacking Hero party will follow. Also, later on when the Ogre attacks the castle to the North-East, be extra careful of the cannon towers defending the gates as they do a lot of damage.