Interstellar warriors are not invincible; there are too many strange and terrifying things in this universe.
Actually, if those greenskins had been a bit more strategic and, in particular, if that leader Tear Meat hadn't been so impatient to lead the charge, but instead kept sending murder cans with ordinary greenskin kids or even fart spirits to consume the ammunition and pave a way, and then had Tear Meat lead enough troops for a decisive, final blow, Matins and the others' ultimate fate might have been total annihilation.
Ultimately, there were too few interstellar warriors, and their weapons and equipment were too light infantry-oriented. If they had had more heat guns, plasma guns, heavy explosives, or micro heat bombs, or if they had traded their combat blades and chainsaw swords for power weapons...
Perhaps the battle would have been easier, and the cost not so great.