Instructions
Organize your units so that they will have the maximum HP possible. This can be accomplished by placing your units into three units with four units in each union.
Use Rush's union twice to summon a golem. The first time you use it, there will be no effect. The summoned golem will fight on your behalf throughout the battle and can prove integral to dispatching the minions that come along with Hell's Gate.
Maintain high levels of HP for each of the other unions participating in the battle. The Gates of Hell has a Pandemonium attack that allows it to bring back your fallen unions and have them fight against you for the rest of the battle. If you have items that can cure enthralled status, they can be used to prevent this.
Use attacks that heal while also causing damage when attacking the Gates of Hell. If you don't have an ability that both cures and attacks, focus on using the most powerful offensive abilities at your disposal.
Attack the enemy groups that accompany the Gates of Hell first. Focus on one group of enemies at a time to maximize your damage. The Gates of Hell will not attack for the first few rounds of combat, so focus on removing as many of the less dangerous enemies as you can during this period.