I did this by myself with 2 controllers in versus mode. Pick Rachel with both controllers. You could also ask a friend to help you out online but it's doable on your own.
Rachel 1 (controller with your gamertag signed in):
Press
twice and during the second jump press
to use a Silpheed. When you start dropping quickly hold
to float.
Rachel 2 (second controller):
Press
+
. Immidiately after this move press
+
. This should give Rachel 1 a small boost up. Keep doing this until Rachel 2 runs out of Silpheeds.
WARNING: Keep
down the whole time with Rachel 1. Otherwise she drops down to the ground.
Rachel 1:
When you nearly touch the ground press
+
to use another Silpheed. Repeat this step twice.
By now Rachel 2 should have at least 3 Silpheeds.
Rachel 2 :
Repeat
+
/
+
as many times as you can and soon you should unlock the achievement. If Rachel 2 runs out of Silpheeds again you should still have one more left with Rachel 1.
The youtube video will show you how it looks like.
EDIT: Comment from
Jay Insomniak :
It's easiest to back your 1st controller into corner, with 2p standing about a sweeps length away.