i got this at the end of wave 9, maybe because they counted the bonus wave? just make sure to keep summoning allies (i summoned none until wave 4) and use your powers as soon as they recharge. stay back and shoot arrows so you don't die. i only spent money (saving up for 15,000 coins) on upgrading my sword warriors to level 2 after wave 3 so i could beat wave 4 without attacking for that achievement. i used them and bowmen for all the waves from 5-9, so it shouldn't be too difficult.