I do agree that the flamethrower starts off slow but it gets a lot better with the right build. the right build is just go everything that sets things on fire except the bullet one and the blazing speed because those feel like a waste. do haste and and fire rate upgrade you can get fast shooting gun and also the run and gun. you should try it and once you take down the boss take the tome of rage because you already do 2 damage anyway with fullisade.