As mentioned in the earlier solution, there is a total of 5 unique bosses that you need to "touch" to get this achievement.
Set up the game in "infinte mode" on normal difficulty.
When you reach the boss, straight away go to the top right corner and park there. Then proceed to touch the boss to make them go manic mode. Don't need to move and just wait, the bullets even at manic mode would not touch you at all.
Simply let it run it course and after a while, the boss would automatically die and you would be credited for the kill. Do this for the next 4 boss with the above method and it would unlock at the fifth boss.
Hope that helps.