I ground out this combo a bit earlier and what I found was to do 5AAA > D.Garu[3]~222 > 5A > 5B > etc.
Basically, you hold 3 long enough to get them going, then right before you go under them or would put them too far away, tap 2 until you land. I've also gotten [9]~[3] to work but it's really tricky and not very reliable. It's definitely one of the more finnicky timings I've run into.
Also, what I personally struggled with when I was trying to get the combo is that after you get the last hit of D.Garu[3] (which is what should pop them up into the air allowing you to continue the combo) was that I was pressing 5A too early after landing. You have to delay it quite a bit to get things right.
Finally, you can cross them up and still continue the combo if people were curious.
EDIT:
Some people are asking if it's character specific, I just tried this on everyone. It's possible on everyone with varying degrees of difficulty, here's a chart and afterwards I'll give some advice on the people that are Hard - Very Hard.
Easy:
Kanji, Labrys, S.Labrys
Medium:
Mitsuru, Naoto, Yukiko, Akihiko
Hard:
Yousuke, Aigis
Very Hard:
Kuma, Liz, Yu, Chie
For Yousuke - He seems to fall a bit faster than the others so you have to be really spot on with your 5A.
For Aigis - She always ended up falling weird. I haven't actually looked at the character weights in the game but I think she's really light. I had to delay the 5C after the 5A > 5B or she would tech.
For Kuma, Liz, and Yu - You have to wait for them to pop out of the 5AAA for them to get high enough during D.Garu and get enough untech time. It's easiest on Kuma, hardest on Yu.
Chie - Borderline Impossible. She's so small that delaying the 5AAA makes the D.Garu almost not hit and then she falls so fast it's so difficult. The best way I've found is to do 5AAA [Delay] D.Garu[9]~[6]~[3] really fast and adjusting from there. She is by far the hardest character to hit with this combo.
EDIT 2+3:
Revised Everything to be easy to read.
Hopefully this helps you guys!