We went the route of SC + MB.
Sch x 2 ( could do with 1 )
Bst x 1 (Bird Pet)
Geo x 2 (can do with 1)
Blm x 1 (Faster with more)
Pld x 1
Smn x 1 (Faster with more)
few other jobs without specific assignments.
Our run we had 11 people.
Silence helps but not necessary.
Pld should be able to tank it with very few cures.
Geos do Focus Malaise Acumen Languor.
Dispel is highly (highly) resisted. including light based.
BST can use bird pet for 100% dispel.
Keeping Shell off genbu is a must.
Schs make
SMN can MB, however ours was used to help in debuffing genbu.
Basically it's the same setup as apex party. Debuff mob with geos. Sch makes skillchain. people burst it. only real difference is that bst needs to dispel mob every 9 seconds.
The bird dispel was the key for us. without it we could not make it. timed out every time.
Good luck!
EDIT: Yes I meant Fragmentation :) Attempted Gravitation for Death. Never procc'd.