Now when you are ready drop your main character down and summon Terramorphous. (Note: You can still do this after Terramorphous is summoned however it is much more difficult.) This character will serve as a placeholder so that when your main character dies you will not lose your progress facing Terramorphous and he will not regain all his health. If you run to this ridge you will see that you can crouch and walk out behind it, completely blocking all of Terramorphous's attacks. Far to the right of the horn you use to summon Terramorphous there will be a small ridge at the very corner of the map before the open cliff meets the rock face. First, get to Terramorphous peak and have your second character drop into his lair without summoning Terramorphous. The second character's gear doesn't matter because he will not be involved in any combat.
To beat Terramorphous if you don't have Xbox live or don't like joining sessions, all you need to do is set up a split screen game with your main character and a second character.