You don't necessarily need to do this on your first playthrough on the level, enjoy it first as this is one of my personal favourites!
Once you have completed it, re enter the level so all enemies reset.
Charge to the end of the level avoiding all sheep fodder (it doesn't matter if you hit the ram enemies).
When you get to the final room, talk to Bartle the Wizard and he will give you an orb and the trophy will unlock!