The final mission of chapter 6 is “Seed a Barren Earth.” It is the 36th and final mission of the game.
Use the BurnSirius mech.
When the mission starts, switch to the Sirius cannons and proceed to shoot up the mysterious NEXT. Just boost and dodge it's fire power and it should be destroyed in no time. On Hard, do the same as on Normal difficulty, but after you destroy the mysterious NEXT, another enemy NEXT will show up. Just do the same to the second NEXT and you should be finished with the mission.