Feed it a purple fuel cell.

The swamp monster can be found on level 3-6.



Have one of your characters operate the bridge lever while the other one does all the grunt work.
First you need to take a blue crystal from the left room and take it to the room on the right to make a blue fuel cell.



Second you need to combine the blue fuel cell with an orange crystal on the upper room.



Finally just take the finished purple fuel cell and drop it in the monster's mouth.