Wait until you get another Mythic so feeding Arden will actually acomplish something noteworthy (i.e. raising your level cap). By that time another option may present itself and/or you might be able to get another Arden to E+ or something so you're not completely left without him.

I only have 3 M heroes, and none of them are Wilders. I have an L+ Nemora and Tasi. Normally it wouldn't be a question, but Arden's the only fodder hero that's worth anything past L160,  and I can't raise the Resonance Crystal level until I get a 5th M anyway.

Should I feed my L+ Arden to one of my other L+ heroes?