As for WA:XF bosses in general: Went over this in chat somewhat last night, but here's the issue to me. Take Ye Olde Godlike boss. Now take the exact same kind of fight but give the PCs some kind of awesome damage move that is spammable and does triple everything else you can do (or alternatively rule this move legal if you saw it as illegal before). Everyone agrees that the relative HP of the second boss goes down compared to the first boss, because the PC party can crank out more damage. Cool. Now, let's take a third version of Ye Olde Godlike boss, but rather than handing out a new powerful damage move, let's hand out Cancel Strike instead. What happens to that boss's respect? Uh. Good question.
Thing is, Rob Turn and Cancel Strike are *at least* as valuable to a PC party as the awesome damage move I mentioned first, and you are guaranteed to have them. So the third variant of this boss should be punished in some way vis a vi the first boss. Problem is I'm not sure how, since arbitarily cutting their speed feels, well, arbitrary. Yet nevertheless the amount of broken should be reflected in some way, similar to how I'm sure many gutcheck FFT boss respect as low due to some of the powerful setups in that game. I dunno. Even games with *very powerful* Haste spells still have to contend with enemy turns which could be used to mess with the enemy party. Not letting the enemy get turns = an infinitely powerful Haste spell?
Snowfire, on the Decelerate/Cancel Lynch Mob, yeah, you probably could try to set up in this manner. Just...means that you need to be using Decelerate on a number of PCs (Given the numerous, numerous variety of things I'd rather have than Decelerate, it's a notable thing), and have a lot of spare MP for Levin to use. Given that I was generally using Levin's awesome speed and move and the ridiculousness that is Blast to pound away on the large number of enemies that generally inhabit bosses' areas, this would actually be a minus tradeoff considering the whole map, IMO. And then, you are focusing on attacking bosses with plain physical attacks which are lackluster damage wise and can miss! However, You do bring up an interesting point on Chelle versus other bosses, but...that still gives her 65% damage or something? Combine that with awesome speed and non-fail durability (at least in my eyes), and I'm not seeing the Middle/Godlike split. Throwing out PB against bosses (And there are some obviously where it's just not going to work-- Zophar/Myrias/Jade/other giant things aren't going to be pushed back) makes me think that she'd be a low Godlike, maybe a borderliner (She'd punish PC laden Heavy, and does fairly well overall against a good number of Godlike PC).
Hmm. PCs should be in the level 50s for all the Chapter 4 bosses (which is what we're considering ranking). That's room for 8 skill slots. If you think that even more powerful skills than Decelerate should be held against XF bosses, fine, but that'll make them even weaker. Notably, some of the stat boost skill slots are pretty powerful - Atk / Def / Eva / Mag / REs +25% are all quite good, and Def and Eva are rather easy to get. Plus they're rather easy to interpret.
Don't want to get into the point too much since this is ultimately personal playstyles, but I only learned Decelerate on two characters - Clarissa and Levin - and that + Slow Down on an enemy was often enough to keep most random mook bosses turn-locked. Didn't take much, but obviously there are other ways to be awesome as well. Re Levin's MP, though, thanks to Decelerate he usually didn't even have to use it much, but it did push things over the top due to CS meaning a boss couldn't take a turn and OHKO an important character or something. Against Charlton 2, for example, after Slow Down + surround to take advantage of Formation Arts, I think I only had to Cancel Strike him once. And Clarissa couldn't even help out with the Deceleration since he nulls Strahl Gehvehr for no apparent reason. (Okay, in fairness, he was Degenerator'd as well, which also slowed him down. But every boss should be degeneratored 'cause it rules.) As for physical attacks being meh... well, Degenerator helps get past evasion concerns in general, and a three-person Formation Strike tends to do as much damage as, say, Secutor's Hard Smash or Fencer's Revolver Sweep, AND often criticals for even more damage. So yeah, while physicals are often meh against the support, I found them pretty good when focusing on one enemy.