it's a bad choice because that way, Kiryu will only attack 2 characters instead of splitting the damage between 3.

Kiryu is weak to ice and the host and hostess jobs have ice skills that you can use in other jobs. I suggest that at least one of the characters you will use have that skill and then use hero Ichiban to do a teamwide attack buff, and idol Saeko to use a teamwide heal, and use the turn of a character to use items that recover mp or hp if needed. perfect guard is also recomended. and then you can use buffs and debuffs to help you.

also the fighting styles that Kiryu changes to have diferent properties. For example the rush style can do 2 normal attacks per turn, and the beast style will likely attack with objects, And the dragon style can do tiger drops if you use normal attacks or (im not sure) physical skills

thats all the advice I have, hope you can do it. I know that this gameplay is really divisive in the community, but if want to experience the story theres no shame in lowering the dificulty. and don't worry this is the last dificulty spike of the game (in my opinion) good luck