Cannon: Get light sloop, put some points into basic shot, find strongest patrolling ship Behemoth or EITC Juggernauts hit it once get away from it and then shoot it from afar at max cannon range until it dies if it gets too close keep moving your ship further back, this can be done solo if you pilot the ship and then hop on cannon
Sailing: Kill with broadsides. There's no trick to this one just need a really strong ship.
Gun: Find high level living enemies use Silver shot, Find high level undead use Steel shot, or the best way since those are high level is use basic shot no take aim and use a weak gun, more hits to kill longer to kill but nearly double XP
Sword: use Sabre only. More hits lower damage same time to kill means higher XP. Weak Sabre will give more XP because even MORE time to kill
Dagger: Throwing knives similar in concept to Sabre
Grenade: There's no real efficient trick to leveling up grenade. Best way is the moment you get grenades go to Kingshead behind the white fence in the marching yard, so that you are still close to their level range when your training grenade so that you still get good XP from it.
Voodoo Doll: Best way to level voodoo doll is just to kill with it, There's no special technique that I'm aware of with it.
Voodoo Staff: using Blast to kill or using weaker staff skills to kill can yield much more XP, but with a longer time to kill. A lot of XP in the game is based on how many hits to kill
Potions: No actual trick have to max it out the normal way
Fishing: Try to get the fish that weigh more but other then that there's no gimmick to level faster
Notoriety: I believe there used to be a tattoo parlor glitch that you could do forever and each time you would drop and re-aquire the quest it would give you 1000XP don't remember if they ever fixed it don't think so.