Rei is required for this battle and starts the battle knocked out, so when you revive him, he may be immediately KO'd again by Mikba. If Rei survives long enough, he automatically transforms into the Weretiger and can go berserk and attack the party unless you use the Influence ability.
Ryu should buff the party with Shield at least twice and then use the Fusion Gene in order to learn Shadowwalk from Rei. The third party member should be Nina. Make sure that she has Influence and target Mikba. Then have her use Meditation in order to increase the efficacy of her later spells. If she has learned Shield from Yggdrasil, have her use it once or twice for good measure. Follow this up by using abilities such as Magic Ball, Simoon, and Iceblast. Heal the party when necessary after Mikba's powerful physical attacks and expect to revive Rei once or twice. Also carry plenty of Antidotes and Blowfish, since Mikba has Venom Breath.
The biggest threat to the party's well being is Mikba's high Reprisal rate, which almost always causes him to counterattack when he's struck. Plus, he tends to get critical hits extremely frequently, especially during counterattacks. As a result, it's advisable to keep a close eye on your HP and heal immediately when it gets below 50%. Since Rei is always attacking, and cannot be manually controlled, he has the highest risk of instigating counterattacks and critical hits, so keep an eye on him. Fortunately, this also means that Mikba will always get attacked each turn, so if the other characters are forced to go on the defensive, Rei's constant attacks will help to still whittle Mikba's HP.
Should you use Garr instead of Nina, make sure he has Influence, and have him use items when necessary, and otherwise deal damage to Mikba. If he has the Focus ability can be used to bolster his attacks in order to deal more damage. His naturally high HP and Defense will also help him to survive several strikes if needed.
As Mikba is a Devil-type foe, a Demonbane in Ryu's hands (and in dragon form) will deal much more damage to him than other weapons. This damage bonus will also carry over to other physical damage based abilities such as Shadowwalk, Super Combo and Double Blow. The earliest way possible to acquire a Demombane is by using Exploration (select the "Distant" option) in a developed Fairy Village through Exploration (Distant) any time after you leave the Dauna Mines as an adult. The Demonbane is not necessary to progress the game, however, besides Mikba it also proves useful later while fighting Gaist.
