You can get this trophy with ease as early as the second bridge in the Lhusu mines, just by running back and fourth until you've emptied the bridge of skelebobs; at this point flee back to the first bridge and back to take on a whole load more skeleton monsters.
Personally though, unless you're trying to level up early, I'd wait until you got to the Nabreus Deadlands. There's a secret area through the swamp. On the map below (external link), if your back is against the orange teleport crystal take a right then hug the right wall. You'll fall into this area which is a haven for skeletons. Make sure you have a decent set of gambits as they come thick and fast.