Doing this fight today I kept getting knocked underneath the floor anytime I missed interrupting Manni's big AoE. It was also knocking NPC allies into floor as well. Manni at one point was below the floor, untargetable, yet still able to shoot me.
I have done this battle twice before and never had this happen.
Despite Mannimarco's meta cheating I was able to beat this on the 5th try. I tried very hard to stay outside the central plaza area where Manni stands, so as not to get stuck. Still got ground into the wall a couple of times but I was somehow able to jump out.
Submitted bug report, hopefully ZOS looks into this. It affects every player.