Ok, here's how you do it. Go install AutoHotkey. After it's installed, it'll create a script in My Documents folder. Edit it, and add the following in there:
This corresponds to the Combat settings in-game (which you cannot change to mouse clicks normally). So, in game, H is normal attack, G is strong attack, etc.
The script remaps LButton and RButton to H and G respectively and then shift to U (hold shift now raises the shield). I also set Xbutton2 (which is the browser-back button) to parry (Y in game), but your mileage may vary depending on if you even have that button.
Finally, Ctrl+Alt+Q will quit AutoHotkey.
So what you do is this: Start the game, spawn your character. Tab out of the game, start AutoHotkey, then tab back into the game (keep in mind your clicks are now disabled so you'll have to use Alt+Tab to enter the game)
Ta-da! Old Dark Souls control scheme.