I recently did Qarn... and LOVED it. To be fair, it's actually the easier of the three dungeons, and the final boss I'd argue is actually one of the easier bosses in the dungeon. It's just dodging, which we should all be experts at by that point and while the laser beam doesn't have an AoE marker, I think it adds to the much-needed difficulty a little bit more.
In my first ever run I only ever got 2 stacks of mummy rot. One from not realizing that the mummies had a 'radius' which inflicted the rot and merely sidestepping to avoid them. The second from the laser beam as I had been expecting an AoE marker before it fired. Once I got hit, I learned quickly and it never touched me again.
