Ok answering a few questions:
Intrinsic, Faimon 1 isn't more exp than hell modes in general. The reason people do it so much is b/c it costs only 3 energy, and often gives back 1-3 more. From mid-way on (I believe Hydeni Ruins) the hell levels are more exp/energy, even considering the energy returns. Faimon gives better mana though too and it's pretty fast, so a lot of people just stick to it.
For your next 5*, I'd probably go wind ninja. That guy is just sick with his superdots.
Kirun, wind and water inugami aren't bad mid-tier units, but yeah they are usually just food for fire/light. Fire is of course a sick attacker, and light is a healer/support with group heals, +attack bar fill, and removes buffs. He can be farmed in the light secret dungeons, and I want to get one this week (and a light bearman!)
Epikion priest honestly are all good. That said, both fire (invulnerable) and water (arena defense) have very specific things they are very good at, so people count them as "best". Neither of them is probably as good of an actual healer as the wind E Priest though, so that's up to you. Wind is still great, it just doesn't have a niche that it's really great at like the other two.
Edit: Also check out wind e priest's "block flow" and get creative. You could do things like combine it with a fire e priest and make your entire team invuln for 2 turns. Or if I used her with my shadow elemental, make his invuln last a turn longer. There are a bunch of buffs that could get pretty sick if you make them last even longer than they already do.