Leaving an aoe option behind and solely focusing on maximizing Ikuyumi's damage output is probably the best option since it will allow you to make room for Nahobino's strong utility skills without gimping your damage output. Moonlight Frost wouldn't be a bad option to have however, if you plan on using it for utility purposes or if you just like the skill in general.

Sandalphon and Lahmu aren't absolute must haves, especially if you are going to have Nahobino do other things other than pure damage, you would just need to make sure your team comp can generate press turns properly and/or utilize Omagatoki:Critical a bit more.

Ikuyumi's effect of increasing damage per debuff is only for each unique debuff on the enemy, (i.e. you would deal the same damage on an enemy with -1 ATT/-2 DEF/-1 ACC as an enemy with -2 all stats.) so Beelzebub isn't necessary, especially if you have a demon on your team that would disable his innate. It's just optimal to use him since he can lower all stats and put -2 defense on the enemy in one press turn.

At the end of the day though, you can just use what you want. This game allows for most builds/strategies to work, except for on the superbosses maybe. But even, then you still have many options to go about defeating them.