how in the world did this guy manage this build with lowering the ap cost with his plasma pistol?

[OH_EO]

https://www.youtube.com/watch?v=i6_4MSDMShY

[Qiox]

He is using the unique weapon called Lemurian Blaster which has a base AP cost of 35. With 16 Dex that gets reduced to 23.

It is not that great of a weapon since it only has 4% crit chance and 100% crit bonus, compared to a crafted one which can have well over 300% crit bonus on the weapon which scales to over 650% with your feats. For comparison, you need 17 Dex to lower the regular 40 AP cost down to 25 AP so that you can shoot twice per turn without any AP buffs.

[Archerodees]

Jesus Christ the sound the crabs make when they pop-up is LOUD AF