In looking more closely at the arena, ranking actually works completely differently than I thought. Offense and defense don't get added up, they're separate and scored separately. Which seems a bit odd since you can't really control how many defenses you get or against who.
The downside to this is that my rank is lower than I thought (although I am getting the second highest tier possible for offense) but we get way more feathers than I thought since they're scored and awarded for three different categories.
Edit: Hmm, there's a fair but slightly unintuitive mechanic in the arena where your chain bonus resets after you reach seven wins. I just got a win on the hardest arena difficulty that raised my base score considerably, but I'd need to get back up to seven wins again for it to actually count. Could spend crests, but... Eh. I'll wait.
Camilla is a great Takumi-slayer if you can get him caught on terrain, bait him out, or get the jump on him with Dance or Smite. Killing Takumi, getting Savage Bliw damage on everyone, and most likely getting one more hit in before she dies is totally worth kamikaze-ing Camilla.
Edit edit: Who all gets Smite, anyway? I've only seen it on Bartre so far, but if he's the only one who gets it then I think it's more than enough to make him meta. It has some significant upsides over Dance (and some downsides too, but at any rate it's a viable alternative).