LEGO Batman 3: Beyond Gotham

This achievement can be done in story mode or free play. 

Playing through the ‘Arrow’ level and you will come to a part where you have to take out a sniper in a tower. Once you've blown up the tower, you will see unlimited enemies spawn out of a tent in the next area past the fence. Stay in the area where you blew up the sniper and as long as you don't zip up the tent you can keep shooting enemies down with your bow and arrow weapon only. Holding down  will help you auto aim to each one; shoot 20 of them and the achievement is yours.

Play through the level until you get to the final area behind the fence. Here there are three tents; two in the first section on one in the next after you blow up the tanker truck. What you want to do is not zip up the tents, because if you don’t, endless streams of enemies will come pouring out making this achievement simple. All you need to do is use your bow and arrow to defeat them. Press  to shoot an arrow or hold  to select targets. Make sure you are far enough away from them so you don’t melee them. Defeat 20 enemies and the achievement will unlock.