iv found terrakion first he killed all my pokemon and the second time i killed him and now its been 3 to 4 months
If you wanna battle Terrakion again the only thing you have to do is beat the Elite Four again. Then it will reappear.
Well because you killed Terrakion you have to defeat the Elite 4 again at least one time and then go back to the cave, Terrakion should show up again.
Hope this helped :)