As you enter the room all of the doors slam shut, a quick check revealing these are magically locked. You look around and realise you are not alone.
With nowhere to hide everything depends on the makeup of your party and the enemy's.
The single most useful character type to have here is a controller, they can alter the terrain to make it more suitable for your own party and less for the enemy, which could completely change the outcome.
The first thing you're going to want to do is keep their melee from getting to your ranged - if you've got more melee than they have that should be easy and as a bonus you'll probably be able to get around to their ranged and keep them busy while your ranged focus on taking out the biggest threat.
If their ranged are trying to take out your ranged you may be able to temporarily neutralise them with a controller throwing up some kind of smoke-screen so they can't see the target. This may force their ranged forward to be able to see, bringing them closer to the melee and easier for your melee to reach.
If you want to keep their melee away from your ranged you could use the controller to put down a zone or wall in front of or around your ranged that'll slow the enemy down or harm them, providing a disincentive to go there. Obviously you don't want to block your own ranged's vision so a smoke-screen here would be worse for you than for the enemy. Also, a zone they can just walk around in one turn may as well not be there, if there's nothing to force them through it.