I've literally been trying the last tier all day today with all g8. Had no problem with the first 3 stages, nearly always got to the final stage with all my protection and ended up getting destroyed every time. I've watched YouTube videos, read posts on forums to get help and tried taking out different enemies while trying to keep Thrawn stunned and saved for last. It was just not working out for me, so I managed to take him out first. I don't know how I pulled it off but it worked. Keep trying.