All maps can be fought well with an AR and BR but here are some of my favorite strategies.
Guardian with shotgun, standing in the lower section of the lift room with it can own. Just watch the points where they can drop down next to the lift.
Valhalla with shottie and sniper combo rocks if you are a decent sniper. Just stand in either of the shotgun caves, or the really weird mound next to the pelican.
Highground depends on the other team. If they dont go for the rocket I would say use the ghost, it is pretty easy to splatter people here. Other good weapons for here are sniper, shottie, even the laser!
The Pit is a great one, try going for the sword first, it is ganged up on less at first then the rocket, then lie in wait for the other guys to go for the sword, it also helps to have a teammate with a ripped turret in there.
Snowbound a lot of fighting takes place in the central underground room, I think your best bet is to lob grenades in the room and use close up weapons. One nice tactic is to go to the big door and fire at enemies across the room, using the door as cover.
Construct is the only map where the flamethrower can be king. Grab one and fly up the lifts to set enemies ablaze. Another great tactic is to take the sword to the top of the lift. Use a BR or sniper to snipe down the hall and the sword to kill enemies coming up the lift.
Last Resort is a classic, I go for the ruins no matter what. Grab the shottie that is near the sniper in the ruins and you can own half the map, plus the shottie works great in the halls of the crumbling ruins.
