To easily defeat Brother 6 with the flamethrower, start by either running a circle around him until he stops shooting fire or hits you with the fire. If he hits you, he will laugh and give you an chance to attack. Use the Dashing Slash (X) to stun him. Next, use the X(4) combo until he shoves you away. After hitting the ground, immediately use Dashing Slash again. If done fast enough, you will interrupt his attack and stun him. Repeat the combo on him, then repeat the entire process through the rest of the phases.