Okay, i've been poking around in the game,  and i've realized that the yellow-ish plague that's under the portal makes it invincible. As i've seen it should be make the mobs regen fast, but due a bug it makes the portals unkillable. However, you can still affect them with skills that causes enemies to be moved, like the crusader shield's number 2 ability. 

So, i've figured it out, if you push the portal far enough from the middle of the yellow puddle, it becomes killable and you can finish the mission. Obviously it's just a band-aid to the problem (that will be fixed fast enough, props to the developer team for the fast patches), but at least you can progress towards completing all the missions.