This Achievement is for beating the "Obsidian Pinnacle" level which can be difficult to tackle. If a visual assist is needed I've included a walkthrough below with easy to use time stamps.
The final boss is easiest to eliminate using arrows and I suggest conserving them as much as possible within the level. The first portion of the battle is simple, using a combination of minimal arrows and hacking that should work by quickly.
The second portion is much more difficult. I suggest keeping a medium distance, and following the purple beams around. Use health only when you're low, and keep arrow spamming. I finished this part fairly quick, but others have noted to me it's quite hard.
1:57:59 Obsidian Pinnacle Level
2:08:55 Arch-Illager Boss Battle
2:09:59 Heart of Ender Boss Battle