Best way to earn Orbs?

Everything: Lunatic chain challenge, Rival Domains, Story, Tempest, Quests, GHB quests, Blessed Gardens, BHB's, Training Maps, Login Bonuses, etc.. There isn't an infinite way to get orbs, these are the finite ways of getting them right now.

Right now? I'd say Tempest Trials, assuming you've completed the story missions, chain challenges, etc. TT nets you 42 orbs in total I believe, which is the most rewarding event in FEH so far. Add that to the orbs we will be getting for the new heroes, and you should have a decent amount of orbs to get a five star or two if you're lucky

Complete all/most of the content the game has to offer. Tempest Trials holds the most orbs per event. Also don't pull as soon as a banner drops, you can try to wait before it ends to keep saving orbs and build up a nice pile.