World 5-Castle (New Super Mario Bros. Wii)
From the Super Mario Wiki
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The player has to cross lava by means of grabbable grate, some of which is round and spins and some that retracts back into lava or walls. Green fireballs from Iggy also attack from off-screen. The boss battle has Iggy with his pet Chain Chomp, which grows after being assisted by Magikoopa. It drags him around the room, and Iggy, who is riding in a seat attached to the Chain Chomp's chain, occasionally shoots spells. There is a platform in the middle of the rectangular stage with two Donut Blocks, and six Jump Blocks, three on each side of the room. When Iggy is attacked, the Chain Chomp glows red and jumps in random directions. When beaten, Iggy and his Chain Chomp fall off screen and Mario progresses to World 6.