Blaster Master Zero

Full Acceleration Blast is an optional upgrade you can pick up in Area 7. It requires staying in one place to charge up for about 5 seconds but will defeat Skeleton Boss in one continuous attack if you can manage to pull it off. I found it best to bait the boss' first tracking blast as you're jumping in the air, then immediately start charging Full Acceleration Blast while that attack is still on screen passing above you. It's not super hard, but may take a couple tries if you get unlucky with his bouncing boulder attacks.

But I mostly wrote this guide for the following content: if you miss this Achievement playing the game through normally, I just verified that you CAN get it during Boss Rush. 👍

Good luck!