so apparently it was first added in Copied Factory and I didn't notice it then, but I noticed in E6N that they've added a new attack marker for tank busters

and that's a great idea!

it's pretty silly that you just have to guess

it's like the stack marker but just one arrow and it's red

Yeah it was for the giant robot. This seems to be for encounters where the boss can’t be moved, so instead of making sure the boss is pointed away from the party you have to make sure the TANK is pointed away from the party.