Mission 98 - Confront Doc Gaston
The Cistern is a maze, with several points of entry and seemingly endless number of criss-crossing passages and extraneous rooms. You don't have to play it like a level of Quake (though that's not the worst idea), but I would try to kill every hostile you meet- you never know how they might circle up behind you, or above or below you later. Also - if you reach the underground sex shrine, you've gone too far.
The most secluded entry point is from the south. Come up the alley and note the lone outbuilding with only one sentry. Take him out and proceed down the ladder here, into the labyrinth.
There's lone guy on overwatch- end him, then down the stairs to take out another two guards. Both other exist here lead to the main room. There's a stack of cash to the right, but the left is less guarded. So grab the cash then go left, entering into a hazy purple phantasmagoria.
The sentry to your right is probably still lurking. You need to get past him- you can either sneak down to the lower level and take your chances there, or take care of him now. Killing will cause a panic, but between the haze and cover, it's unlikely you'll come under fire immediately. If you really want, you can duck back the way you came until the heat dies down.
Either way, bear right, on your right, you'll see a loading dock. Use the railing as cover to kill the hostiles, then down the stairs and nearly straight east up a short flight of wooden stairs, annihilating as you go.
Left up some tile stairs and now to your right you'll see a doorway surmounted by curtains. This is Doc's Lair.
Walk in, shoot him up (along with any guards that hadn't already joined the fray), and score a brand new racket.
Collectible Alert: There are Vargas paintings, one in Doc Gaston's subterranean lair, and in the restaurant upstairs (at street level).