Okay why -Def on A!Camilla?

Yes, but she already scores low so the difference shouldn't be that much

If your worried about arena red flier duel exists so if you have a Laegjarn or whwever you pull one she will score 170 points

Let's face it. There really should only be a handful of units you'd worry IVs for arena scoring. The rest of them should tailor to min-maxing. Def is her dump stat. Preserving her HP allows her to do extremely well vs mages, especially on EP with her base kit.