While the game is mostly easy, there is a huge difficulty spike when it comes to facing Kakaro. In this battle, you will be unable to use Kids, but you will be able to deploy all your commanders. Hopefully you didn't make any of them disappear in Kakaro mode because you will need all the manpower you can get.
To start with, give all your NP to Kabachahn and Patton. If you have any items that increase special move usage, give them to Patton then to Kabachahn if you have spares. Equip Rick, Hero, Dikacis, and Barbazza with range increasing items so that they can contribute with their high attack power once they run out of specials. Equip one of them with the item that increases range by 2 and decreases movement by 2 and counteract it with a Nimble Feather so that you deal more damage per turn.
Kakaro's main body has 14444 HP and he has a long range attack that deals big damage. He is helped by four hands with medium range, two mouths with long range, and a brain that performs a summon that deals damage to all your commanders. Killing the body parts helps negate the damage but you should mainly focus your attacks on Kakaro because the body parts revive the turn after they die. He also comes with 5 Kakaro pawns that have medium range and have decent HP. He summons 5 of them every turn.
Kabachahn should be moving forward and be protected so that he can use his special. Patton should be leading the charge and be placed next to Kakaro's body, with two fighters behind him. Your long ranged units that can deal damage should be used to hit Kakaro and as many body parts as their specials allow. The rest of your commanders should be used to protect your fighters and long ranged units and to deal damage to the body parts(ideally with specials that can also hit Kakaro's main body). Nanari's special is very useful as she can heal your fighters and Patton after a few specials.
When the most commanders possible can hit Kakaro, use your Dark summon before you attack. This allows you to deal double damage with your normal attacks. It doesn't work with specials, but since Patton's allows him to attack 5 times, he will deal thousands of damage to Kakaro.
If all goes well a few commanders will still be alive to deal the finishing blow on Kakaro.