Story-related, can't be missed.
In the first wave the pumpkin will be shooting out three flaming balls constantly at you. Try to dodge your way around them and he'll now stomp on the ground and zap you. Again jump over these. Now is the part where you need to throw your followers in his mouth, you'll need to do this three times.
In wave two, he'll start puking out pumpkin seeds out a few times. The pumpkin will start doing ground pounds like he did in round one, but this time he will be moving so you will need to go under him while he's in mid air. His mouth will be open so you will be able to throw three of your minions in his at his mouth, but now the catch is that he will be moving around the place.
During wave three he will do a spinning attack. To avoid this attack run at the wall and wall jump off it and hop over him, he will do this a few times and then he'll open his mouth. Once you manage to throw in three followers in again, he will be defeated.