Instructions
Step into the Volcano warp gate before heading up the volcano to face Megatron. Go back to Autobot HQ and equip the Bulletproof, Slapshot, Deflector and Aftershock Mini-cons. Return to the volcano and switch to vehicle mode. Drive off the ledge toward the platform below and transform into a robot directly before you land. Otherwise, the fall will take some of your life.
Use the Deflector Mini-con to block Megatron's energy bullets when he's walking towards you in his robot form. When he begins running towards you, run away from him while jumping to prevent him from grabbing you and dealing damage. Shoot him with the Aftershock Mini-con while he chases you, periodically dropping grenades on him with Slapshot. Megatron will stop in his tracks, stunned, when a grenade hits; use this time to either distance yourself from him or attack him.
Watch for Megatron to turn into a tank and then start running around him in circles to avoid the missiles he shoots. Although you may have time to hit him with a few grenades, consider saving the Mini-Con energy for when he runs at you. Use this easy-to-dodge attack instead to weaken him slowly with the Aftershock cannon.
Continue practicing Steps 2 and 3 to learn Megatron's attack patterns: when it's best to defend and when it's best to attack. Megatron's health-bar in the top-right corner will slowly drain as you attack. When it eventually empties, you have defeated Megatron.