Need BBEG stats / CR help

[Mazlith]

Please help me out creating amazing encounters in a RtR side quest. I would love to have your best and most hilarious ideas. I also need you to recommend stat blocks for the encounter.

If you want a 'lil backstory & what I have planned:

In my RtR game, the PC's will be finishing Nkechi quests this week and returning to Elder. They have 2 in game days before the expedition begins. I decided to reach into each characters back stories for some side madness. The faction they chose is the Red Mantis. They would love to get revenge for freeman's warehouse attacks, and they think the Shackles Pirate faction is behind it. They're right.

In Corky's backstory, his rival is a Pirate Captain named Black Sam. As they return to Elder, they'll catch sight of Black Sam's ship parked next to the Shackles Faction's ship. Corky the Drool will want revenge for his permanent drool. So I'm going to have them goto Nkechi's favorite pub. Where Gelik will be performing his ballad about "Gelik: the hero of smuggler's Shiv, and the PCs' half orc barbarian is a big oaf, with his semi useful companions, who'd be nothing with out the great Gelik." Kyras, the ex-indentured spy/ninja, is going to be confronted by an agent of his past and will not want his whereabouts revealed.

As they leave the pub it should begin with chase scene of a spy/ninja type person leading to the Aspis warehouse. As the chase begins, Aries and her all female pirates ambushing PC's for romantic revenge. Possibly kidnapping the PC and taking him to Black Sam's ship. If not, then there faction leader (Red Mantis) will order them to take out Black Sam's ship because it's filled with supplies that will empower the Shackles Faction's expedition. This fight will go simultaneously with the chase scene.

The PC's:
Corky wizard4
Bronn fighter4
Kau Kau witch4
Ulbak barbarian4
Kyras ninja2/summoner2

THE ENCOUNTER Here is where i could use help, ideas & CR recomendations

BBEG encounter with a pirate named Black Sam (+ a minion or two). How would you stat this pirate based on the PC's levels for a great fight? How could the ship explode destroying the ship and allowing the PC's to escape. What treasures might he have?

[Mazlith]

on a side note, this makes me want to buy the NPC codex

[EvilMinion]

Have some pirates with a home-made still on the ship, making rum. Then just need for it to get damaged during the encounter.
Nothing like a still in a confined space to give you options for a nice kaboom!
And far more fun then the generic 'black powder' explosion.

Perhaps have an alchemist on the ship, with a full on alchemical lab, for a similar effect but with more options for varied results... perhaps everyone near the explosion (pc's included) gets hit by some random alchemical effect (thunderstone, smokestick, acid, alchemical fire, tanglefoot bag, et al) as the alchemists store of materials all goes up at once. Lead to lots of fun chaos during the escape!

Then again, chances are you wouldn't find these things on a ship... so perhaps move them to a structure right beside the ship.

Or a local entrepreneur, with a small skiff, who sells his wares from his boat to ships in harbor (like a food cart) who just happened to be working the area at the time.

On purpose or by accident, explosions can be fun!

Wait, there was a supply of fireworks on the dock waiting to be loaded onto another nearby ship? And they caught fire too? Uh oh...

[Mazlith]

The average party level is 4.

What should the recommended CR's be to make this hard or challenging?

[EvilMinion]

Depends on if you want them to kill the pirate, or just mess up his boat and destroy his cargo (I recommend the later, since I love reoccurring bad guys and it helps build on the rivalry between the PC and NPC, rather then just ending it abruptly so early on)

You can easily do the later with a higher CR pirate who they are not expected to take on directly. Nothing like blowing up a guys ship to make him worry about other things than smiting intruders. Guess it depends on if you think the player would be happy with screwing said captain over... or if he'll only be satisfied with the captains death and will not wait for this gratification.

As for CR's ... that's what the gamemastering section of the core rulebook is for!

Just remember the pirate is the wizards nemesis via backstory so his skillset should be a significant part of the victory.