You'll earn a mulligan when you kill 3 goblins with one shot. So you'll earn plenty playing this game for gold medals. (just shoot any tower or wall with goblins).
Using a mulligan is when you've shot the catapult and afterwards you put your left arm up for a few seconds, giving you a second chance to improve the shot you just made.
It's basically a rewind for your last shot.
do this 50 times to earn the chievo.
It might come in handy as well, when you messed up your shot going for the gold medal.