A useful strategy against them is to cast Reflect on one of a party member, then reflect Thundaga spells off of them. Aqua Breath, Tsunami, and in the Game Boy Advance release, Flood, all bypass Reflect and hit another weakness of the Armored Weapon.
Umaro equipped with the Snow Scarf and Berserker Ring can absorb the damage of Absolute Zero and Magitek Laser while being able to deal some decent damage in return; his Tackle can easily punch through Armored Weapon's relatively high Defense of 190. In the back row, with Miracle Shoes equipped and the random castings of Absolute Zero healing him, Umaro should be able to win this battle easily. Mog might fare even better than Umaro, with his high Defense from being equipped with the Snow Scarf and a Thunder Shield or Paladin's Shield.