Mercedes SP Build.

When I first tried this in the town of Magatia, it worked just as you said. But when I tried training with it in nearby maps, it didn't double cast. I returned to Magatia to try it again, but it still didn't work. Any explanation for this?

EDIT: Hmm, I think I found my problem. It appears that Booster has to be on for it to work.

Yes, it requires Booster.

@Joe: I can link it with charge drive, but the thing is that doing that will not allow charge drive to rush all the monsters to one side. Furthermore, charge drive must be pressed immediately after Gust Drive (during the GD animation), so I can't delay the skills a bit ala Dual Blade's Flying Assaulter -> Tornado Spin.

I'm thinking, perhaps getting more jump will delay GD (more airtime) enough to give CD the space of time necessary to rush all the monsters properly. I don't have jump equips though, so can't test it. =\

edit: tested by jumping off platforms various times. GD-> CD does seem to work properly when GD is pressed high on the air (the GD animation kind of gets stuck in the air for a moment and CD activates when the character touches the ground). So I think it's probable that getting more jump will allow people to do GD->CD properly. Which pretty much turns Mercedes into a Dual Blade without the high damaging assaulter. High kick demolition makes up for that though...

After Gust Drive, you have to spam Charge Drive for it to activate correctly and it doesn't allow me to do this with a macro hotkey for some reason. Otherwise, you get a huge delay if you try and don't activate Charge Drive correctly. You can push the Gust Drive macro immediately after jumping, and it will activate when you reach the required height, but macros tend not to work if you're pushing any other button at the same time, so try not to hold down the arrow keys while you're jumping, or if you are, spam the pomegranate out of the Gust Drive macro.
After Rolling Moonsault/Leap Tornado, if you hold down left or right, and spam Gust Drive's macro, you can change directions in midair.
Gust Drive also links into Lightning Edge, and Lightning Edge links into Charge Drive, and they should execute faster because of this. I haven't researched how much delay is cut out though, but I'm looking into it. I didn't actually know this until a few minutes ago and I'll try it out on KMS to see if it's anything special.