The Droids/Creeps are the little ''Minions/Ennemies'' that spawn infinitely with a small green health bar (That can be kill in 2-3 hit) Yoiu will need to kill those a thousand times. This will come naturally if you are going for the other achievements. You can take a look to your stats in the menu.
Simply go the Help & Options > Stats > Creeps : Number of Creeps killed.