Shackles is great in the early and midgame while the game is still not super trivial. I dont plan a build for 15+ i plan it for 1-15 basically because after that you are an immortal god anyway. Executioner will not give u +4 AP if u kill 2 enemies at once. It will only give you 2 until next turn regardless how many you kill.
I have started another solo campaign with Sebille with all this in mind. Finesse was a hell of a lot easier in the early game Compared to INT. Id recommend starting out with a rogue/ranger build and respec into this on the boat. Struggling to get to lvl 4
Hm, maybe my english is just horrible, but I will try one more time.

The 4 AP came from using 2 different abilities each of them should kill at least 1 enemy soe ach of them will give you 2 ap 2+2 = 4. :P
Getting to level 4 is super easy in Fort Joy even without Deatfog barrel, do stuff like the fight with the elf at the gate (Ifan and the elf help you) and quests where you don't even need to fight like yarrow girl, Wither, freeing the elf (here you get another super easy fight with Silence trying to kill the lizard).
You also get a lot of exploration xp if you go into areas where you wanna come back later for fights like the frogs.
(You can even go into the swamp super early, just sneak by the guy who behaves like a churchpreacher and converts an elf into a silent monk on the left side, you don't even need invis for this.
(This will turn the magisters hostile but the effect is not that much different then removing your source collar and they can be easily avoided if you don't wanna fight them.)