The Spasmosaur appears in the Triceraton Tri-Sports Arena. He will often spit at the player, trying to damage them. He will also try to catch them with his tentacles and then throw them up in the air. He will also try to smash them with his tentacles in different ways. To damage him, attack his tentacles and he will drag them into his body. Then strike his nose and he will get furious. He will rush into a wall and take heavy damage. Do this a few times to take him out. He will sometimes make a u-turn and go back to the middle of the arena before he has hit the wall.
Game Boy Advance
Strangely enough the boss fight takes place in a cave on the planet D'Hoonnib with the player controlling an aircraft. The Spasmosaur shoots projectiles akin to the Triceraton Cruiser, then will follow up some attacks with strikes from its tentacles. Just let loose with shots to the Spasmasaur's forehead to take it out.