Complex sentence: He was first seen in the beginning of the episode with some helpers of his, which are some of the Kaiju-esque monsters you always see in Meta Knight's flashbacks, in a battle against Knuckle Joe on his home planet.
Split into simpler sentences: He was first seen in the beginning of the episode with some of his helpers. Masher 2 is in a battle against Knuckle Joe on his home planet.