Later, Buddy

Kamina, Simon and the rest of the Great Gurren Brigade plan to take down Thymilph, one of the Four Divine Generals of the Spiral King, and his massive mobile Gunmen fortress, the Dai-Ganzan.
Their plan involves having Simon using Lagann to take over the Dai-Ganzan the same way he fuses with the Gurren, but an something unexpected causes a great tragedy in the following battle.