In my most recent playthrough, I'm getting some surprisingly solid usage out of using a few oddball support characters as major attackers. Observe this video of my fighting the 18F boss at about Reimu lvl 90:
Part 1:
http://www.youtube.com/watch?v=_RnKMjOaRPA Part 2:
http://www.youtube.com/watch?v=e9iKFNJYP_Q Want to know who did the most damage in this battle? Would you be willing to believe that it was Sakuya, of all people? Yes, she alone blazed through 18F's first form and dealt 215k damage on her own, then went on to do a total of 720k damage throughout the course of the entire fight. Second place was Youmu, who is by no means a bad attacker (and is actually pretty high-tier, I'd say). Youmu contributed 660k damage total for me, but she had elemental advantage on her side against the Wind and Nature forms, doing a total of 485k damage to those two thanks to their 2x weakness to her attacks. Sakuya, on the other hand, had to do all her damage without any elemental weakness on her side, yet still came out on top. How in the world did this happen?
First off, Soul Sculpture is terrible for boss fights, as its formula isn't very good and it has horrible delay. No, Sakuya's real weapon is Killing Doll. Killing Doll has a very unique damage formula; it has pathetic ATK influence, but also has an almost unheard of 550% damage multiplier. Usually, Sakuya's ATK ends up getting swallowed up by the enemy's DEF, leaving very little left over for the damage multiplier to use. However, once you give her an ATK boost, Killing Doll's damage output jumps up tremendously.
As an example, Komachi (whom I tweaked for maximum offensive output in this party) was doing 35-40k damage against 18F boss's neutral form with Scythe that Chooses the Dead. That's pretty good given its relatively low delay for a power nuke. When buffed with 1 Thundercloud Stickleback, Komachi does 50-60k damage instead. So a single one of Iku's buffs translates to about a 50% increase in damage. Now let's look at Sakuya and her Killing Doll. Without buffs, my offensively-built Sakuya can manage about 20k damage with Killing Doll at level 85, only half of what Komachi can do. After a single Thundercloud Stickleback, her damage jumps to 45-50k, more than doubling in power and almost catching up to Komachi's attack. That's a bigger proportional boost than pretty much any other spellcard formula in the game, which shows just how important ATK buffs are for an offensive-build Sakuya. While Thundercloud Stickleback often translates to a 50% boost in damage for other characters, it means a 100-150% jump for Sakuya.
Now, 45-50k damage per attack after buffs at this point in the game is pretty respectable, but it might not sound phenominal to you just yet. That's almost comparable to how much an unbuffed Silent Selene/Slash of Eternity/KO in 3 Steps can be expected to do. But there's more. Sakuya, who already has good speed, has a mere 48% delay on Killing Doll, making it easy to spam without switching out. All right, that's sounding better...but then you get to The World. A 100% SPD self-buff to start things off? Yes please! After buffing herself, Sakuya can expect to get off at least 3 Killing Dolls in the time it takes Yuugi to recover from a single KO in 3 Steps. And thanks to her solid HP/DEF/MND, you can expect Sakuya to stay in and take whatever the boss throws at her (she survives Start of Heavenly Demise with no problems).
Right now with my current party, I almost never use Luna Dial in boss fights anymore. Sakuya is spending all her time buffing her own SPD and then spamming Killing Doll like crazy. She has certainly surpassed all my expectations as an attacker, that's for sure. Although I should probably give some of that praise to Iku as well. But then again, I'm not sure if my opinion of Iku can get any higher.
