For Scarab specifically...

Something like Miscreated with your 3 turn shield going before Visix (A1 only on boss), Stag (speed down/def down), Armiger (preferably in Destroy at some point, A1 only) and Coldheart should absolutely smash the boss. If you need to have someone faster than Miscreated for some reason, you'll want a Bloodshield accessory on them. 

Arena is going to be a bit tougher. I don't really see anyone I'd want to bring up to improve in Arena right now, but I could see Toragi being a great option (CB/Dragon) along with Visix and Armiger.

As to your question about which 5-star champs to use as food - I assume you are excluding those who are already level 50. I think there's definitely some questionable decisions on who you chose to make 50, but now that they're there, at very least leave them for faction wars teams.

The three at level 1 5-star you can feel free to turn into food, though. Given you have Skullcrown at 60 already, I doubt you particularly need Fellhound as a campaign farmer.

Who was questionable for level 50? What things or champions should I choose to be level 50? Where do I get the information from  for future decisions? 

The easiest way to answer all of your questions together is - you shouldn't level anyone to 50 that you don't plan on leveling to 60. And you shouldn't level anyone to 60 without already knowing exactly what team they'll be on.

I'd hazard a guess that you don't have the answer to that last question for most of your 50s.

Very succinct answer here. Champs like Dark Athel, Seducer, Elhain, Athel and Dilgol fit into this category.

Hellhades is a good website to get information about champs. Or the forum here.

The problem with some of your level 50 champs, like Elhain, Athel and Dark Athel, is not, that they would be bad champs in general. The problem is, that they don't add anything to your account. You allready have a lvl 60 Galek, a lvl 60 Kael, a lvl 60 Dark Elhain. All these champs have similar skill kits, they are damage dealers of magic affinity. There is not really a place where you would use the lvl 50 champs mentioned. Maybe in faction wars.

Thank you. I will re-adjust moving forward.