I just got this achievements the other day. On the last level on the extreme, I would take the boss down to his last breath and THEN get the target markers. For some reason when you run out of T-ENG, you don't die like in most levels, so you can't die without getting hit. Anway, take the boss down 99% of the way, then get the markers, then hit him one last time to kill him. That way you don't waste time getting all the markers just to have the boss kill you - and then start all over again. Sometimes the last boss hits you with cheap hits and you die without a chance. This'll save you time but making sure he's going to die, then ignore him and get the markers, then kill him.