Most Obnoxious Levels in Gaming

[Mag]

Dragon Airie in Dark Souls 2. The drakekeepers have infinite stamina and cannot be stunned, making this place basically impossible for non-ranged builds. In NG+, these guys can basically either 1 shot you or leave you with like 1 hp.

Then again, there's plenty of obnoxious areas in the Souls series, so it's not that surprising.