Okay, I think he ratman annihilation is definitely doable. I tried what was suggested with Durin and it did kinda work. I still lost part of my team, but it was enough to hold out until the boss was dead.
The problem was that I was too weak to fend off the remaining fodder right after the initial attack, so 4 of them made it through while I was waiting for redeployment cooldowns.
I think I just need bit of luck, or better yet, a tiny bit more damage output from my melee ops. AoE would be good, or maybe even arts.