For this, you'll need an Arsenal (Optional), GC (Other skills are fine too, but GC gets it done fast), and BreakFall (Trust me, will make it a lot faster)
1. Make a room, preferably solo. (Newbies usually get in the way)
2. Select Lv. 172.
3. Start the game.
4. Call out your Arsenal.
5. Protect your Arsenal as best as you can.
6. GC to keep enemies in place, so that Arsenal can shoot.
7. When you kill all enemies, DO NOT RUN TO THE BOSS.
8. Stay at the beginning of the bridge, align with the arrows, and shoot repeatedly.
9. Bubbleboy will get pushed, and you will win.