To get this and the 3000 kills in one battle trophy very easily all you need to do is do the following. Remember not to kill Guan Yu until you get 1000 and 3000 kills
1.Pick the Battle of Fan Castle-Wu Forces on beginner
2.Kill all of the enemies and offers apart from Guan Yu
3.Now go to the top left of the map where Guan Yu is. Enemies will keep re-spawning behind him. Get the rest of your kills here.
The trophy will take a few seconds to unlock.