When playing FFA games and you don't have a team who is there to help you, the most important thing that you need to do is to become aware of the enemies near you all the time. This is due to a very obvious reason that no one is there to tell you about the location of the enemies near you.
One of the best tips that I ever heard for FFA games is that you should play as an observer. By becoming an observer, you will be able to locate the weakest players and take them down first. Medium range weapons like DMR will help you effectively steal kills from other players. The strategy that I often do is to look for two players who are engaged with each other and wait for the right time to strike easy double kill.
Dying is not a big deal for FFA games, but always remember that dying has a respawn time and you don't want to waste a lot of time just to respawn. You should not worry about your K/D unlike your TS. In you are playing FFA, you always need to be in action, and this will include you dying.
The best thing that you can do to minimize attacks on you is to stay on the edges of the map, which may not be applicable to some. This will prevent people from attacking you from behind, and will allow you to see all the enemies who are running towards you. This will allow you to focus on fewer sides of the map, while forgetting about the attacks that will come from behind you.
These are some of the tips for Halo: Reach that I can share. Just keep these things in mind, and soon enough you will become one of the best players of the game.