Kamiizumi is usually a defensively-oriented foe, often using Swordmaster abilities to defend against attacks and prepare counterattacks. Nothing Ventured halves damage from physical attacks and counterattacks for twice the damage of a normal attack against any character that attacks him physically. Before Swine halves damage from magical attacks and counterattacks for three times the damage of his normal attack against any character that attacks him with magic.
Know Thine Enemy picks a specific target and counterattacks that target for four times normal damage should they attack him. As Kamiizumi's physical attack already deals an extremely high amount of damage, any one of these counterattacks will likely be fatal against its intended target. Occasionally, usually after a counterattack has downed an ally, Kamiizumi will Brave twice, launching three attacks at the party.
Kamiizumi's counters usually come first and are difficult to predict, so a good strategy is to Default a couple of turns and then have a member of the party killed, doing so will cause Kamiizumi to Brave twice and attack with normal attacks three times and then leaving himself unable to use any counter for the next turns. Using this chance to revive and heal party members as well as attacking is the best time to deal damage to him. A White Mage is recommended, since it can quickly Raise and heal any damage done by Kamiizumi, also if previously Defaulted, even a revived character will not lose their chance to attack. Monks and Rangers can be effective offensively but can pose a risk defensively should Kamiizumi deal enough damage to potentially kill them, although Monks have higher HP so they have less risk.
Eventually as Kamiizumi loses HP, he will Brave twice regardless of whether he KO'd a character or not. If this happens then the player should do as usual, revive, heal, and attack while Kamiizumi cannot do anything else, and otherwise Default all the time until a new opportunity comes.