I am pretty sure you have to kill all Controllers in the area as they are responsible for you being "tossed around". Once all of them are dead, you will have a small window to get to the portal though.

From my experience it's not so hard as long as your aim is good. :)

Well its a stupid gameplay idea if you just need to enter portal but kill all enemies before that. And arent they respawning all the time?

What Valo said, if Controllers are alive you get booped from the portal.

After the save point, 4 are alive and 4 more spawn when you get near the portal. Plus, 2-4 spawn on a reinforcement timer. Don't waste time shooting them down with rockets, use the Revolver and the SMG. It's definitely possible before the first reinforcement wave arrives.

You need to enter the portal then it turn blue