Against 18F, Iku should be able to handle staying in the 2nd slot full-time to pass out buffs. I used her for both my last two runs of the game, and with a bit of effort she was surviving all of 18F's first form attacks at level 85. Past the first form, almost nothing should seriously threaten Iku if you've focused on raising her MND. If you're worried about how to use your characters, check out some of my videos; I've used pretty much every character you've got on your team for this fight.
ATK Komachi is actually quite beastly for the battle. At just level 81, she was swinging for 50-60k+ per hit after a single Thundercloud Stickleback, and well over 100k+ on the MYS form, which is awesome given the attack's delay. Give her some affinity gear, and she can survive anything the boss has with no problem. Reisen is also not a terrible damage dealer by any means. Just because 18F is immune to debuffs doesn't mean she's useless; on the contrary, after self-buffing she should be doing 60-80k per hit on the elemental forms she can exploit, and she does so quickly thanks to her massive speed buffs and decent delays. Nitori of course just murders everything with Thundercloud Stickleback for backup.
Keeping Sanae alive is really quite challenging, though. I really can't bother to even try to get her to the point where she can survive Start of Heavenly Demise, so I used her solely to switch in, heal, and switch out immediately, just like how I'd use any other fragile high-delay nuker like Nitori.