Judging from his duel with Cobb Vanth, I'd say something like "At the start of each enemy's turn remove 5% TM from all other enemies and for each enemy affected gain that much TM". In addition if Cad Bane is in the leader slot and not the ally slot, at the start of a battle remove 10% TM from all enemies and gain offense up for 3 turns which cant be dispelled.
In GA: whenever ally BH defeats an enemy, CB gains bonus turn and during this turn his stats are doubled(tripled if he is a leader) and he deals additional basic attack after he uses an ability.
Plus, zeta on basic: Gun Slinger
Deal Physical damage to target enemy twice. First shot can't be missed always is a critical hit if possible.