None of Gehenna's moves can inflict debuff statuses. Right Arm, Bane and Sweep deal physical damage (Bane also inflicts Sap). Instead of using Mind Crush to counterattack, Gehenna makes use of Comet. The General also has Blindga to make physical attackers less useful, and Tornado to put one character into Critical state. His two most powerful multi-targets abilities are Soul Purge and Dark Star. Gehenna absorbs Dark-elemental attacks, but is weak against Light.
The player should switch off all Dark-elemental weapons, and use Holy, Divine Shot and Final Heaven to severely damage him, especially when his HP is relatively low. Phantom Rush, Midareyuki, Gigaflare are also good.
Since this boss can only inflict Sap and Blindga, which doesn't affect Final Heaven, Phantom Rush, Midareyuki or spell-casting, one healer is sufficient while the others keep striking the general. If the party is at low level, having two healers is recommended.