In ffa, try to avoid jumping into fights. Set up off the the side with a turret and rocket pod (preferably) then unload grenades and rockets while grabbing reloads whenever you can. His rocket lvl1 is by far the most useful in ffa, easily can clear maps if you're clever with it.
Lvl2 isn't as good as it looks but can still be pretty effective. Also always stay by your turret and keep distance from your enemy. Neutral square is a good move to create distance in a pinch.
Foward Square into the shotgun level 1 can work as a hit confirm
Up throw into up square x3 is a nice combo that can be extended depending on the context: - If you have the shotgun equipped, you can follow up the last knife hit by jumping up and hitting them with up triangle - If you're close to a wall, you can jump up and hit them with an air up square x3
Emmett has a few AP burst combos, but they rely on him being by the wall: (Rifle) Forward square --> Down Triangle --> Up Square x3 --> Jump --> Air up square x3 (Shotgun) Forward square --> Triangle --> Up square x3 --> Jump --> Air up square x3
Actually landing forward square can be hard though, since it has a long start up time.
Try not to abuse the turret too much, as it gives other players AP when they hit it. Ideally, when you set a turret down, try to protect it. More often than not, at least one other person will drop everything to go destroy the turret - see who that is and use the fact that they're focused on attacking that instead of you to your advantage.
You can also use the turret as a distraction. Want to go get a shotgun/rocket launcher safely? Try popping a turret down to take some of the heat off of you. Want to lure people into a certain spot to set up a super? Again, the turret can help with that.
Another thing to make note of is that his default rifle loadout actually is useful. The pistol has decent AP gain to it, and while the rifle on the ground is mostly useless, doing forward triangle in the air makes for a great anti-air poke - it'll interrupt most anything thanks to its wide spread.
Emmett's good at camping and fighting up close and personal. Just see which style is best suited for whatever situation is on hand.