Mighty No. 9

The achievement description is wrong. You need to attack the last boss with every mighty number ability and also kill it. I'm not sure if it's essential to hit only the final form with every ability but that's what worked for me.

I highly advise doing this on normal difficulty with 9 lives. Obviously you must learn all the bosses movements and how to kill both forms safely (brandish and battalion is all you need for first form). Use battalion on the walking plant phases and dash through when she's weakened and purple. On the standing still plant phase this is where you'll want to use brandish to cut down/ dash through the 2 stacks, then when it's immobilised you have the opportunity hit it with any ability you wish from up on the first platform.

Seismic and Pyro are two of the more difficult ones to hit him with so its nice to get these out of the way.

Credit goes to Awoo.

If anyone finds some more precise details I will update the guide accordingly :)