First off, make sure to pick up the medal during the fight. Second off, this will be difficult depending on what difficulty level you are playing the game on.
If you are playing on Hard, a horde of rats will spawn every now and then making your job much harder than it needs to be.
Okay, you can use your fists or slingshot to damage the boss as long as it's not flamethrower.
Equip Platform and as soon as Sewerjunk spawns, go near it and put the platform between the boss and you, and keep punching him until he goes down in the water.
When you get to choose how many tentacles it has, pick 6. This will save you a lot of trouble trust me.
Rolling under it's "bullets" does you no harm so keep this in mind.
Later on when it's HP is lower, the boss will destroy parts of the terrain as well as stick his tentacles on the pathway. When he does that, punch them a few times until they retract, this will make space for you to place the platform and keep punching him again.
Do this until his HP is empty, and when the game asks you what happened with the flamethrower, choose the second option.