To go against Marco Barbarigo directly is suicide. Luckily you're packing heat now.
Go through the Carnivale events to get the mask.
1. Ribbon stealing is active only on the blue clusters on map. Careful you don't steal from other citizens or you'll alert the guards and use up valuable time.
2. Racing is straightforward and unlike other races mostly on ground level.
3. Capture the flag is easy once you find a pattern but is annoying if you lose the flag to your opponent. (Thank goodness there's only one.) Note where your opponent's base is and you can better cut him off and steal it back, if needed. Once you get the flag it's almost 100% smooth sailing since your opponent doesn't chase you very well as opposed to his running away from you. ;p Just don't get lost and you'll do fine.
4. The fighting event is a piece of cake if you've mastered disarming and dodging. First 3 opponents are standard but then your next batch are cheats and come at you with blades. Give it back as good as you got and you'll come out on top.
Win all the events then steal the mask back from Dante using the ladies (do have the sense to steal from behind).
Once you've crashed the party, keep out of sight of the guards by blending with the guests and ladies. The guards will patrol so you may have to keep moving as they would disperse your cover. I found this annoying as it's hard to immediately find new cover before the guards notice you. My advice is see if you can anticipate where they will move by their red dots on your map and move yourself before they arrive.
A timer will count down to the time you can finally see Marco Barbarigo. But he's on a ship away from the party floor. This is when the courtesan mistress suggests using your newly acquired gun to reach Marco by bullet, masking the shot by fireworks.
There is a group of courtesans right next to the railing across from Marco's boat. Blend in with that group, target Marco, cock your gun, and shoot. (Aim and time your shot carefully, patrolling guards may block your shot and take the bullet, causing all hell to break loose. Also, the longer you aim, the more suspicious surrounding guards will be of you, so take heed.) Bring Marco down, leave the party, and you've won. Congratulations!
The only thing I'm miffed about this mission besides the guard-avoiding is I can't seem to time a shot to coincide with a fireworks pop as the fireworks are all softer than the gunshot. So unless the escort mother is only conjecturing, if anybody knows how to muffle your shot, post it here.
UPDATE 10/5/2011: If you do cause a panic, you might want to try what GREENGORGON did: use crowd panic to his advantage by getting onto the ship in the hubbub and killing Marco up close. I haven't tried this yet myself but it is an extra thing to try if you don't want to restart from checkpoint. (Read his comment, there's a novel in the making there ;>) Thanks, gorgon!