How useful? Very useful. Canceling Fatal Strikes can start a completely new combo.

Preparations:

Get the Taunt/Taunt 2 skill for Repede
-Buy Tama-hagane [Capua Torim]
-Synthesize Tama-hagane+1 [Tama-hagane, 2 Grimore Pages]
-Synthesize Tama-hagane+2α [Tama-hagane+1, Tolbyccian Water, 2 Boar Furs]
or Tama-hagane+2α (lol namco typo) [Tama-hagane+1, Tolbyccian Fish, 2 Basilisk Scales]

Get the Bark skill for Repede
-Buy Takemikazuchi [Nordopolica (Or find in Atherum)]
-Synthesize Takemikazuchi: Flare [Takemikazuchi, Flare Stone, 2 Bird Feathers(Medium)]

Set other Player and Repede in your party, and set Player to Semi-Auto, Repede to Semi-Auto or Manual.

Initiation:

This is the method using two controllers. It's easier and is seamless at best, but it requires uh a second controller. If you can do it by yourself, good for you. This is the method Nore used in his combo exhibition (vol. 2).
Prepare the Fatal Strike. For the sake of example, I'm using Yuri and activating a Green FS using Divine Wolf: Gale. Just spam Dragon Swarm, Brutal Fang, Ghost Wolf, etc, to empty the enemy's green FS gauge. Once it's empty, activate Overlimits. The higher level, the farther Gale's wind blast goes, so use level 3 or something. Use an Arcane or Altered Arte and activate Divine Wold: Gale. At the end, the Green FS should show. Initiate it, and immediately go into the menu as soon as Yuri starts. Now, at this point, canceling it here would keep you where you're standing, and you want to fly forwards, right? Unpause and pause when Yuri starts to dash. Get the other guy ready on the other controller. As soon as you unpause, taunt with Repede. Now, there's a 30% chance that the Bark skill will activate, canceling all movement. Yes, this means it will only work 30% of the time. After you practice, feel free to not open the menu and just taunt with split-second timing. If you did it right, Yuri should slide forwards as if he was using his FS.

Yuri should be in the neutral position, meaning you can do anything, even while sliding. This cancels artes and resets counters, so you can start new combos and stuff.


This is the method using one controller. While more difficult, it's possible to do by yourself. This is the most likely method you'll be using, because you have no friends.
Prepare the same way as you would above, except slap Raven in your party, and disable Yuri's Taunt skill if he has it. Use Divine Wolf: Gale, and activate the FS. Pause while you're moving. Switch Repede to Auto mode, and switch control over to Raven. Hold the left shoulder button and push down on the control pad; this should switch the strategy to Take it Easy. Take it Easy causes everyone on Auto mode to Taunt, including Repede. Once again you have a 30% chance of the FS canceling and Yuri flying forwards. Exit and bring the menu back immediately. If Yuri's stopped attacking, it worked. Switch control back to Yuri and pound away.

Example of the one-controller method


FAQ

Q: Help me it's not working bawww
A: stfu

Q: Is there a way to have Repede Bark 100%?
A: Not that I know of, and probably not.

Q: How do I use Divine Wolf: Gale?
A: Get out of my topic.

Q: So you can use it with everyone else?
A: Except Repede. He's hax enough anyways.

Q: Can you use it without Overlimits/BA?
A: The example only used OVL because of the Burst Arte. No, you can use it with other stuff too.

Q: I have friends, you suck.
A: You're on this forum and are thus a liar.

Q: Does it have to be Fatal Strikes?
A: No, it cancels all enemy and player actions, so even during artes it's possible.
A [common] usage is using Ghost Wolf and canceling that instead, but you're going to have to make room in a combo for that to work. It also requires even better timing. Also, because Judith's Green FS sucks, canceling New Moon Blade or Luna Ascendant will work as well. Also also also.

Q: Can you cancel other Fatal Strikes?
A: I haven't tried, but blue ones can be canceled. You would go flying upwards like Nore did in his vid. I'm not even sure why you would want to cancel a red FS.

If you are looking to submit it to TSF you may want to touch up a few areas like the FAQ. Though that's just my opinion. Also good job on writing a guide. Kudos to you jiggly.