Note: To get the decent ending you will need to spare the old man's life by killing only the guardeyes in the Grand Church, located in Evrai. The battle in question is located after you fight Ray and complete the dungeon in the church. He will be able to fly your town and use that to seal Infinity at the end instead of Ryu following in his mother's footsteps.
I begin this video from the last save in the lower reaches of Infinity. I choose to go with the party I already had which consisted of Ryu as the lead fighter, Rand for muscle and healing spells; Bleu for awesome magic attacks and buffing; and finally Jean makes a nice support character to have around. I exit this room from the right side and then go north from the point where the warp point deposits you to arrive at the final boss.
At this point Evan decides he wants to mess around with your party. After some necessary plot advancement your character decides to cut Evan's hand off ushering in the Deathhevn form. At this point you are alone and must cast the Anfini dragon form to resurrect your party.
After your party joins back up with you, you engage Deathevn for the final battle. My tactics involve two separate phases, an offensive attack and the defensive rounds depending on the flow of the battle.
At the beginning of the battle I concentrate of having Ryu blast Deathevan with the G. Dragon form to start chopping away 999 points of damage during each turn. I like to do this at the beginning of the fight because the boss likes to get nasty later by draining your HP and AP with his "Become God's Energy" move. When he starts doing this move it is best to abandon the AP dependent offensive moves and resort to attacking. During the G. Dragon blasts you will see the striking order of your party members. This becomes useful because Ryu can unleash the blow during one turn and then you can have the remaining members after him use w fruits to start replenishing his spent AP during the same turn.
During this first round I have Rand attack the boss when he isn't healing or helping replenish Ryu's lost AP. Bleu will start by raising the Attack power of Rand and then Ryu. This helps raise damage if they counter an attack, which is likely to happen especially with Ryu. Jean is pretty much there to hand characters items to recover AP to resurrect a downed ally. He has decent speed and defense which makes him a good choice to stay ahead of the bosses moves. Bleu assumes a more offensive role, casting Bolt X whenever she doesn't have to pass out AP items or buff the attackers in the group.
As the battle grinds on I tend to gauge when to switch from offensive to defense by how many AP replenishing items I have left in my inventory and how often Deathevn is using his bone laser and HP/AP sapping attacks. When he becomes more aggressive, I stop feeding Ryu AP items, as those are best used to keep Rand's AP up for healing spells. Ryu seems to do just fine on his own as an attacker, provided bleu is able to renew his attack up status with her spells. At this point in the battle I like to have Rand follow up every bone laser blast with a heal 3. Bleu can switch back to bolt.
After the battle the ending plays through with the good ending and the credits.