Hmm... I seldom experimenting with this Devour move so there's not much I can say.

Initially to obtain that ability you need GF Eden in Ultimecia Castle if i'm not mistaken (i beat the game a very long time ago...)

Luckily I've jotted this down during my play. Hope you'll find it useful.
HP:
-Cactuar - fully healed
-Rinoa in Adel +10
-Ruby Dragon +10

Strength:
-T-Rexaur +1

Vitality:
-Adamantoise +1

Magic:
-Behemoth +1

Spirit:
-Malboro +1

Speed:
-Pu Pu +1