SAO Fatal Bullet: Skip directly to Underground Weapons Plant boss
Making a video of this since I couldn't find one when I was trying to learn the trick myself and it would have been handy to have a reference.
I DID NOT FIND THIS TRICK MYSELF. The only reference I can find of it is this guide on GameFAQs by "Arkia": https://gamefaqs.gamespot.com/ps4/217398-sword-art-online-fatal-bullet/faqs/75678/walkthrough
Requires Gun & Sword (which conveniently unlocks just before you reach this dungeon), Vorpal Strike 2 & 3 and Dual Orbital 2 at a bare minimum - at least to use this method. The written guide documents a couple of alternatives, which I have tried and they work but aren't viable for speedruns.
Rough instructions
1) Take a running jump facing roughly the direction I am in the video.
2) Just before you start falling use Vorpal Strike 2 then Vorpal Strike 3 followed up by Dual Orbital 2. The Vorpal Strikes should be aimed straight forward while you may need to curve the Dual Orbital 2 slightly towards the left wall.
3) Use your first UFG shot to scale as high up the left wall as practical while also making a little more horizontal distance.
4) Use the second UFG shot to hit the far wall and flip up onto the platform. This shot will usually need to be close to even vertically to land, so getting decent height from the first shot is important.
Things like using Speed Form skills and/or accelerating to a good speed before jumping off do help a little but I've hit the skip while actually being overweight (with no buff/runup). UFG technique is the most important part by far.
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