Originally Posted by Buruma
It really depends on how you intend to play the game. If you want to take your time reaching level 50 and kill him by yourself then you should probably wait until your second playthrough but there are a few methods for defeating him on your first run. Bring in a few high level friends to help, boost your character to level 50 in someone else's game and hop back into your own, etc...
So wait, what level is this Terramorphous at? I'm level 35 and just got to the Vault's location, done every side mission so far. I knew I was gonna have to get to lvl 50 at some point but I at least wanted to get the side mission achievement first so I could start on the other character achievements..