Taken from the xbox360achievements.org, this worked perfectly for me:
You’ll find Oberon sitting in the middle of Evernight forest, surrounded by a giant tree with a protective shield. To deactivate the shield, you have to destroy the six root nodes that power it. You need green and blue minions to access all the nodes.
Directly behind you as you face Oberon.
Outside the entrance of the Viridian Caverns, protected by a poison plant field.
To the left of Oberon’s tree is a path protected by poison plants. At the end of the path is a large central area containing another node.
Exiting this area through the door controlled by the wheel with red spokes brings you to a platform by the water. Sweep your blues across the water to turn the wheel, extending the bridge. This allows you to access Skull Den. At the end of Skull Den is another node.
Leave Skull Den through the exit that was previously blocked by the root from Oberon’s tree. The path directly upwards and to your left will take you to the Mother Goddess Temple. When you get to the end of the Temple, you’ll have to kill a pair of trolls before you can destroy the node that you find there.
To the right of Oberon’s tree is a path that leads down to a river. Use your blues to move the block in the middle of the water and go down the river until you can cross to the other side and destroy the node you find there.
The actual battle with Oberon is relatively straightforward. There’s a mana pit and brown minion spawn pit to the left of Oberon, so your horde shouldn’t be a concern. Begin by sweeping your minions up the ramp at Oberon’s feet so they can attack him. Watch out for clouds of dirt appearing near you. When you see then, move immediately because roots will shoot out of the ground, damaging you. Eventually, Oberon will summon monsters, beginning with blue toads and working up to bloody unicorns, trolls, and the mace wielding minotaur-like monsters. You can overwhelm the monsters with your minion horde and keep replenishing it from the brown spawn point, but I found it much easier to blast the monsters with my fire spell and constantly restock my mana at the mana pit. In between the waves of monsters, keep sending the minions up to Oberon. Once they’ve finished him off, you’ll see a cut scene and then the ramp at his feet will collapse, allowing you to walk up to and strike the killing blow.