My thought on Shiki's Fusion is to treat it like a Limit, since the Fusions are in no way affected by Neku.
Shiki can access her Lv1 Fusion reliably by Turn 2, and arguably by Turn 1. I don't think her Lv1 Fusion overkills, but one round of her physical chain , plus a second round of her physical chain+Lv1 Fusion is pretty damaging. Enough that I can see it being a pretty reliable 2HKO in Light.
From her second turn on, she can pull out a physicalchain->Lv1Fusion whenever she wants. This just means she needs two turns without Chansey healing to finish it off. In four more turns, she can have her Lv2 Fusion, and eight more turns will see her to a Lv3, which should OHKO Chansey at full health, though honestly, I'd bet that the Lv2 is enough.
My reasoning is that Limits are not reliable damage, they are going to be OPB 90% of the time. Even in a three-turn average, only Shiki gets her Fusion early enough to effect the average. If you throw out Neku, this makes her damage pretty decent. If you allow Neku his pin set of broken, then yes, her damage is pretty lousy, but her Fusion lets her be at least a little competitive. My personal view is to only allow storebought pins and their evolutions (which is still pretty broken, but somewhat reasonable) and hold Neku's damage to amount of damage that a single pin can do in one attack (generally this means until its usage bar is depleted, but some pins have forced pauses between attacks that cannot combo together). I do not allow Neku to combo separate pins together.
It's a somewhat artificial way of comparing Neku to the Partners, but considering that the Partners operate on a completely different battle system from Neku, it seems like a fair way to compare them. For example, Neku's enemies move much faster than a Partner character's enemies, so it seems wrong to try to force both battle systems to compare the same amount of damage by time. So I try not to think of them in damage per second, which seems to be the popular method. Instead, I enforce a Tales-like combo limit on Neku's damage, and consider him somewhat fast. The Partner characters are held to a more Tales/ChronoCross hybrid system where they are allowed to follow a panel chain to its logical finisher panel to attempt to build up their Fusion gauge, and consider that a single turn, along with considering them about average speed, though possibly a little slow compared to Neku. Since Fusions activate instantly, I considering them as being tacked onto the end of a turn once a Partner character builds up the appropriate amount and decides to use it, somewhat similar to how Chrono Cross Elements tended to be used (and Eternal Sonata's special moves, but no one plays that).
I still seriously do not understand the view for Partner Characters not getting their Fusions, since it is only a Flavor-based argument, and if you're using Flavor-based arguments to prevent characters from using their Fusions, then you're pretty much holed into not allowing the TWEWY characters to do -anything- since the game is pretty explicit in the plot that nearly ALL of their abilities are tied to their partnerships (except Joshua, but yeah). It's like... why are you picking on Fusions as illegal? It's one of the only things that makes the Partner characters interesting, too...
/rant
EDIT:
I have a question about how people view damage and averages for single-PC RPGs like Parasite Eve (and I suppose Ys, Crisis Core, and Zelda could be considered as well).
My instinct is to consider all stats as average... And to possibly view average damage as the average of what each individual damaging skill can do, which would create an artificial damage curve where the character's best damage is always decently above average. A little unfair, but when you consider that Aya can't use her best damage but maybe once in a battle, it evens out. (I believe the same applies to Link, Adol, and Zack too.)
Which reminds me: Tonfa! Stat topic Ys Origin and include Ys6 Adol!
-Djinn
