How To Defeat Mollusque-Lanceur

After breaking all four seals in Seaside Kingdom, head to the center of Bubblaine to battle Mollusque-Lanceur. The battle will begin with Mollusque-Lanceur throwing exploding shells at you. Dodge the best you can, capturing one of the Gushens in the water as you do to help in this fight. Mollusque-Lanceur's head will be covered in lava, so use the Gushen to clear the lava off the top of his head. After getting rid of all of the magma on his head, his head will become vulnerable for an attack. Quickly hover over the monster, and then leave the Gushen and jump straight on his head.

After landing the first blow to his head, Mollusque-Lanceur will start retreating from you, still launching spiked shells has he does. Capture the Gushen once again and keep pace with the creature. Do the same as before, using its water cannon to hover over the head of Mollusque-Lanceur. As you do, quickly leave the Gushen and land another jump to his head as Mario. The final phase will be more of the same, except that the monster will be even faster and shooting a variety of exploding shells this time around. Do as before and land the final blow to his head to defeat it.