We went through so many troubles there. Had to redo the unit four times. Then I just took an elevator downstairs and halfway realized that two tank commanders and berserkers are going to wait me down there, because all the monsters reset.

I hope I'm ready to try the whole thing again.

You have to complete the mission in one session, don't use the right teleporter on the startmap.

Yeah, I figured that out already.

But even when we started over (from the first map), it didn't work. I guess the problem was that all our mission items saved too so the objectives were going crazy because of it. Just a guess.

I don't think the presence of items in your inventory is a real problem for the mission. But you may need to retake them each time you try the mission, because it could fire some triggers when you get them.

Maybe you can try this mission in regular mode with your teammate, just for understanding where is the problem.