I made a map of where to stand. But basically, if you stand in that spot the whole time the Ghostly Hero will start his boat ritual and none of the White Mantle will come and attack you. So you can run the level for people and at the end you won't have to fight off all the White Mantle to protect the Ghostly Hero. I'm not sure how accuratetly you can find the point just by using my map but it's somewhere around there. You don't go on the dock...there is sort of a slope you have to go down to get to the dock...just stand at the bottom of the slope. People don't know about the glitch, so no one really runs the mission...So you can make a lot of money.
Even if you don't wish to run the mission...you can do this and you don't risk everyone dying on the last part of the level. I know it's pretty pointless if you wish to fight the whole mission and what not, but it's a possibility, and a glitch in the game.
Lol, I'll get a screenshot as soon as possible. Guild Wars at my house is broken.

I've tried everytihng and it won't install. But I will get one as soon as I can.