To unlock the My Brother is an Italian Plumber trophy in Borderlands: Game of the Year Edition, you will need to kill an enemy by jumping on its head.
The easiest enemies to do this on are the pup Skaggs, as they won't be trying to shoot you back whilst you attempt to do it. You can find of these enemies outside the gates of Fyrestone. When the Skaggs attack, kill them all except for one pup, shoot/melee attack it until it's health is as low as possible without actually killing it, and then jump on its head.
The jumps do very little damage, so this might require quite a few jumps for you to get it done.