Best use for DC

That depends on what you've got.

Personally I consider Sheena and Camilla for my DC. Because Sheena is easier to merge than Hector and I thought about an interesting build for Camilla which includes DC.

If you know about heros of yours, who could shine with it, go for them.

Ayra is really not the best DC unit. Her res sucks.

Try Sheena, Myrrh, Gwen if u can get her res up, or LA Eliwood. I like DC to be on my rounded tanks, since so many DC users like Xander excel only in one stat: either res or def.

If you couldn't decide from those two, save for future unit? I have DC to these two and kind of regret it. Seemed like a good idea at the time.

Someone with enough res and spd to survive mages

I gave it to my ayra but its only use for survivability in challenges paired up with my B lucina