So the room has 3 portals opened to an elemental plane of ice and cold winds that are spewing into your world.
The cold is so extreme that being exposed to it deals damage to each players. Either a player or an NPC companion will cast a forcefield that is one banana in diameter around the player party protecting them from the extreme cold as long as they stay within its circle. The caster is able to freely move around the map with the forcefield following them as long as the mage can continue the concentrate.
Either the caster of the forcefield, or another player, can get within NEAR range of a portal and roll INT to close the portal with X hearts of EFFORT.
Oh by the way, each ROUND enemies spawn from each of the open portals that run through the forcefield to attack the party. Protect the caster because if they drop, so does the forcefield. Shield wall formation!
Of course the elemental plane could be anything, Fire, thunder, poison, etc. Id suggest having the forcefield caster be the one to close the portals so they still have an action each ROUND besides keep the forcefield up while the others fend of the enemies.