Apart from Mythic Raid, Arena rewards are probably the next best. Arena chests may contain hearts, dragon hearts, rune essences and the latest orichalcom, so they're a must have for upgrades. Let's not forget that more doppelganger queen essence is doled out for higher rankings in arena too. I hope that by showing you my arena formation (credit to LEGlON's Viceroy, Kitty), line up and how I prioritise tagging in arena, you will be able to improve your scores for better rewards.

EDIT: GoddessOfHope pointed out that you can tap or click on the top bar of heroes to tag them. Thanks Hope !