"Finish a match with more kills than deaths"
This mission is rather easy to achieve, even without preparation. One easy way to complete the mission is to simply defeat an opponent and then hide yourself for the rest of the match. This method best works when playing in big maps with excellent hiding spots like The Rocket or Eastside District.
Of course, you can also simply play as you normally would, as by the time this mission appears, you should be capable of sustaining yourself against your 7 (or less, lucky you) opponents. This mission should not be of any hassle to you.