This achievement can only be earned in the first story chapter, Prologue.
Ah, another chuckle worthy and simple secret achievement. To unlock this bad boy, load up the first story chapter, 'Prologue' in story mode or free play mode, as it can be earned in both, although I could not get it to unlock while playing in co-op.
You'll be able to unlock this achievement right before the chapter ends. The level is composed of essentially four phases: fighting orcs, fighting Sauron, climbing up Mount Doom, and entering the volcano. Your opportunity in that last phase, when you enter the volcano.
Towards the end of the level, you'll fight a ton of orcs on the mountain, and a group of them will come from a pair of double doors. Once you defeat everyone and enter the doors, you'll be inside the volcano. Press and hold and select the fifth circle going clockwise. From here you can select ANY character; Pick Isilidur. All there is left to do at this point is jump off the bridge into the fires of Mount Doom. If you're in story mode, simply pick him from his normal place in the character wheel.
A word of warning: if you walk all the way across the bridge to the platform at the end, the level will end and you'll need to replay the level again to get the achievement.