For The King

The sea king is massive and marked on your map once you’re far enough along in the “into the deep” campaign. It’s level 10, 760 hp, 12 physical armor and 4 magic. It has two tentacles 285 hp, 4 physical and 8 magic armor. All immune to stun and water. Sea king immune to lightening as well. They will use attacks to damage your boat so have the strongest ship you can buy (or the pirate ship from completing all the option pirate dens) and some boat repair kits on hand. The sea king and the tentacles have to be defeated to win the battle and you get the achievement after the battle.

This is supplementary to the specific information posted by Sir Spink

For most of the bosses in this game it is very beneficial to get a weapon with Reset as an ability. This allows one of your party to constantly use this attack and bump the enemy back in the timeline. We had two such items and the Sea King never attacked once we had destroyed the tentacles.

At present the Sea King is the only boss we have used this tactic against, (plus all the scourges on the way), but I shall copy this to the other boss achievements if they work there too.

Intelligence
Book of Healing has the spell Time Warp. (3 rolls)
Fire Staff and Great Fire Staff have the spell Fire Wall. (4 rolls)

Talent
Archlute, Barbat, Fire Lute and Theorbo have the skill Rubato. (3 rolls)
Blunderbuss and Golden Gun have the ability Warning Shot. (1 roll)*

Awareness
Dragon Blade and Hooked Sword have the skill Disarm. (3 rolls)

As far as I am aware they all require perfect rolls to Reset the enemy, but at the levels obtained when reaching the final boss this should be okay. We did use a focus on most attacks we needed to guarantee. (with two characters rocking Reset you have a lot of leeway).

Please post any other weapons with a Reset ability below, or any corrections to the above.

*I have never used a gun, so not sure if you have to reload between each attack.