To make sure you get all mid-boss achievements in one playthrough, make sure you choose slash mode. The reason for this is because origin mode doesn't not contain stage 5, thus preventing you from earning Armored Helicopter. All stages have a mid-boss and as you complete stages, you'll notice the mid-bosses becoming stronger and stronger. All you have to do to earn the achievement is to kill the respective mid-boss. However, keep in mind that mid-bosses can time-out thus preventing you from earning the achievement. A time-out happens when you don't kill a mid-boss in a certain amount of time. If this happens, the mid-boss will fly off-screen never to be seen again. Therefore you will have to restart your playthrough for a chance to obtain it again.
To prevent this from happening, made sure you have bombs because they do massive damage. To use a bomb press . You can also make sure that you have katanas or steel orbs which also deal massive damage.