You really should aim to choke boss CEXP to Sirius or even others like Palla. Sirius's main reason is his CEXP gain is kind of limited with all the unpromoted mooks still running around.
As for Cain... Not getting doubled and doing 9 damage as a Cavalier... I think only a MU can do that. I can't view spoiler tags on my phone for some reason, but doesn't Cain have like 11-12 Spd? If so, Luke needs two Spd procs, but I'm guessing it puts P-8 into a 10 turn, unless I missed something.
You only really need 1 HP, Speed and Def, the HP and Def prior to the chapter, the speed at least during. The Def is the only really annoying part. If you happen to be powering Luke early, then this shouldn't even be a problem outside of crap luck.
The other thing is Ryan needs a point in Strength, or Luke does. If either gets it, then cool. The real difficult part is Fighter MU surviving a Myrm and an Archer. I was Def blessed (like level 7 with 6 Def, or level 6), but my build wasn't intending for it (I tend to go speed future rather than present, opting for Mighty present instead. Issue being that I'm not 100% sure I had the actual stats to back up the claim). If the usual Def is 3, then I think I had enough...
How to do it: Maric and Ryan attack the Armor over the river, Luke attacks it from the space north of it with an Iron Lance (thus the Str proc form him or Ryan, Ryan preferred). Shiida does her usual thing with the Myrm. Athena takes the space just 1 north of Maric, with MU a space south of Maric.
MU does super effective damage to the Armor, Luke kills the myrm (getting his level off it, for me anyways since I was powering Ryan) and takes a hit from the archer (thus why the Def and HP).
Athena kills the north Archer, Luke occupies a northern fort that lets him attack Cain the next turn while downing a vuln. Ryan kills the Knight (thus why the Str is preferred on him rather than Luke). MU moves face first against the Myrm and downs a vuln with Steel equipped, Maric blasting with fire. Shiida takes to a fort near Cain.
The Myrm kills himself, then the Archer attacks.
Next turn, MU with Maric or Ryan clean up the archer, then Shiida and Athena deal damage to Cain, Luke coming in for the kill (now thinking about it, the speed isn't necessary if you're feeding the kill to Luke, nor is the vulnery). Otherwise, the kill can go to either him, Shiida or Athena. There is your 3 turn.
As for Sirius, I recall dondon saying something like Sirius isn't exactly long term or something to that effect. You could if you want, but it's plenty of trouble. As for Palla, her problem merely is that she doesn't have a Wing.
Besides that thought process, I'mma give Linde a spin this time around. Wanna see what her level will be for it, because if too slow for Aura she could always use Rezire if that would work instead.
That does remind me of something though. If one is to seriously use her, there is always the option of just buying the Spirit Dusts from the base shop and then throwing it at her. Early on, we have but a forge to spend money on, and if it's +4 then we got plenty more to get all 3 dusts and then some. Silly? Of course, but it's also out the gate with 29 magic damage. On top of insane chip, it lets her one shot some lesser durability enemies like thieves, hunters, mages, etc, or it lets her syphon back 18 damage with Rezire if so desired.
By the way, anyone tried Horseman Arran for chapter 1? Works surprisingly well, thanks to enemy AI being rock stupid in that chapter. Also, is it agreed that MU gets the Bond Drop? Because he totally should.