keep the epics and renegade, use the rest of the rares as food. 

armiger is definitely worth bringing to 60 at some point, but that point isn't now. you want him at 200+ speed, 200+ accuracy, and 100% crit rate to be effective so if you can't hit those numbers, he's not going to be showing his full potential. he's also not particularly effective for the parts of the game you should be focusing on at the moment. as early as you are, you should be concerned with dragon and clan boss, as well as grinding food in campaign. armiger is excellent for fire knight, spider, and several doom tower bosses, but not all that useful outside those areas. leave him at 50 until you're further on in the game.

Just to add onto that - the short answer is "never use epics as food". It'll be a *long* time before you're at a point where that's worth doing.

Thanks for the clarification! Good to know about Armiger too.

I didn't think so but I just wanted to make sure. Thanks for the reminder!

To add onto the value of keeping epics:

Even when you get to a point where you're swimming in dupes, you're gonna want those for epic Faction Guardians, most likely. :)

Faction guardians? I don't know what that is...

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Go into Guardian Ring, then on the left side there's a tab for Faction Guardians. The info tab at the top left of that page will explain it in depth, but the short version is duplicates of champions for each rarity will give stat bonsues to all champions of the same rarity, within the same factions only. Over time you'll end up running into duplicates, especially at rare and epic. 

Thank you!