Please check the tutorials before posting.
http://www.staredit.net/index.php?tutorial=81If you are not sure which method you should use in there, then maybe you could share the aspects of your map and we can help you choose
Latin, if you do many many kills triggers, like soldier recommended, do not use Exactly but use Atleast. That way if your kills go straight from 2 to 4, you still get the Atleast 3 Kills bonus. Just make sure there are no preserve triggers on these triggers. SCTrigger is a program that makes these for you.
The last reply you did should work fine. You most likely did not delete your old trigger, which is adding the gas nonstop. Make sure you add hyper triggers if you use the score method.