A first place mission requires you to charge at the most populated spot on the map, with the most effective weapons you know. If you know a machine gun best, use that. An assault rifle with clays provides strength and protection from flanking. It does no good to charge blindly and die quickly. There are two keys to ending up in 1st place. Scoring the most, and scoring most often. It's not just getting a lot of scores, then getting hit, the time it takes to get back on spot will cost you. You have to be able to back up, pull off a few tricks to survive, then get back on spot. The opponents will start to react to you more, and will make a more concerted effort at getting you. This is when you need to be able to feel an oncoming rush, and use whatever tactics will help you diffuse that rush.
You also have to be aware of the QRF people, who will be coming after you as you get more scores. When you're on top, everyone wants to knock you off, your team wants to be there, and their team doesn't want you there.
Even a sniper that only takes a shot once every 30 seconds can end up in 1st place. If that shot is effective, and scores each time, and if nobody figures out where the sniper is and gets him.
The key is not making a lot of scores, but making sustained scores. Get 3, get hit and have to run back is a middle of the pack finish.
Watch the routes and moves that the player in 1st place uses, and follow him a few times. Cover his flanks, and see how much better he does. Once you see all of the elements that go into a 1st place finish, you will start to end up on top more.