I had run into the same problem, the tank being almost impossible to use for the same reason you describe. I tried killing the horde of undead for awhile but that didn’t seem to advance anything. I used the mortars nearby to kill the giants but they kept respawning as well. I almost gave up after about 30 minutes of constant fighting and a few deaths, but then it finally updated and went on to the next part and I was able to complete it. I’m guessing I had to kill a certain number of giants and then kill a bit of undead afterwards, not sure what exactly made it update.
So to work around the tank bug, I’d suggest using mortars to kill some giants (maybe about 6-8 or so?) and kill off some undead and hope it updates. Good luck!