To unlock the Can't We Get BEYOND Thunderdome? trophy in Borderlands: Game of the Year Edition, you will need at least one other player. You can do this via online or split-screen co-op.
You can find the Arena in the Arid Badlands, to the left of Fyrestone. Enter with your co-op partner and make sure that you are both on different teams and then press ready. You will then be transported into the Arena. Win the three-round match and the trophy will unlock.