Beyond Border

This can only be done in the 4th stage. You need to get the crystal turret that costs 44 gold. It's a bit random since there's a chance it won't appear. It only appears in that stage and a crystal turret costing less won't count. It you do have bad luck and not get it, don't finish out the stage. It's the last stage and if you finish, the game ends and then you will have to start over from state 1 again in a new playthrough.

Stage 4. It’s the 44 dollar crystal turret.