Simply equip your Flame Thrower, and Lock your weapon on an Up-Right angle, and while standing on the ground so that your Flame is hitting under the boss's chin. For the second part, repeat the trick, but angle your gun where your Flame is hitting the monster's nose. You'll hit everything that comes outta his nose, as well as hit his mouth when he opens it. The only thing you then have to do is worry about any worms dropped on the ground from the monster.