Arena

For round 7 you'll be fighting 2 spawners, the one on the left will spawn zombies, and the other will spawn snakes. Kill the spawner that makes snakes, and leave the zombie spawner alive. The most it can spawn on the field at once is 2 zombies, which take 3 hits to kill with the level 1 lightning spell. You can farm this spawner for kills once you are done. These achievments unlock midgame, so once you hit the correct number, you should know.