It's not that pliers shouldn't be used, it's that they are very easy to misuse. If everyone tightened the bulkhead with just pliers, then I'm sure several people would have overcompressed gaskets which would result in a leak down the line. I hand tightened only and was able to get it down to a very slow drip. After researching it a bit, it seemed a lot of people were using channel lock pliers to give it another quarter of a turn and that's it. It did the trick for me. Tightening it past this point is where I think you'd encounter problems.
If you haven't already removed the bulkhead to clean the area, I'd try the quarter turn of the bulkhead first.
