I know exactly the choke point that's troubling you. I had the same problem. I let one of the party hang back down the hall out of danger. Let the other sneak just up to the corner around which the pack appears. Just 1 or 2 of that pack would come after the one at the corner. I'd immediately run back to where the other was waiting and take on just 1 or 2 rather than the whole bunch.
The Skeleton Mages are tough, but watch the pattern. Every once in a while, they stop and hang for a few seconds. That's when they're the most vulnerable.
