Here is the full list of all 22 B-12: Brantisky Mk. 12 achievements.
Reach a coin-bank balance of 100+ coins.
Get all six B-12 upgrades.
Spend some coins at the Maintenance Pad.
Destroy a single B-1 unit.
Destroy a single B-2 unit.
Complete the switch tutorial at the hub.
Get destroyed.
Complete the first set of three levels.
Complete the second set of three levels.
Destroy a single B-3 unit.
Destroy a single B-4 unit.
Destroy a single B-5 unit.
Destroy a single B-6 unit.
Destroy a single B-7 unit.
Reach the maximum of 999 coins in the bank.
Destroy a single B-8 unit.
Destroy a single B-9 unit.
Complete the third set of three levels.
Complete the fourth set of three levels and beat the final boss.
Destroy a single B-10 unit.
Destroy a single B-11 unit.
Defeat the final boss.