A neat little trick is this:
In the swamp map, everyone's rushing as a group to separate swamp devourers. If you know the enemies are killing the devourer on the other side of the portal, use it four times first. So basically just go in, return, go in, and return. You would've used 4 of the 5 times before the portal closes. Thus, when the enemy group is finished with their side, only one person can come through, and they'll find themselves trapped among your teammates.