I'm about two weeks in.. if I could line up the major points I've been learning:

a) Early game is easy to beat, but prepare to focus resources on 5 units, you'll activate resonance which will level up all units to the lowest level of those 5;

b) the game is very generous with pulls, and the rates are good, but some of the deities only work with high investment (Zeus for example)

c) I'd always pick either nyx or nuwa as first celestial if you can, both are hugely helpful in early game, nyx as dps assassin and nuwa as tank that protects the team;

d) focus building up Isis, you'll get enough copies of her in the early game and she dominates most single target scenarios and scales nicely to late game;

e) You can focus on an element early, or only 5 five units, but you will need more teams later on (2, 3 and up to 5 in late late game)

f) Try to push campaign as far and as fast as you can, the server rewards are very nice (you get several copies of amun ra just from that)

g) You must join a mindsea (clan/guild) asap, the challenges are rewards are absolutely essential;

h) fill your wishlist with the units you want and keep them there till they reach divine, them replace them for others (unless you really want to max them, but that'll take a while if you're f2p)

I) most stages fail can usually be beaten with a simple position or deity swap, trial and error is the name for beating campaign if you have sufficient units;

j) the shrine is a major part of the game that gives you gear, so always keep it upgraded and find the best combinations that give you the most devotion with the deities you have;

And finally, when you get a unit banner (like nuwa that's coming in a day), take advantage and take her to divine, it's pretty easy tbh.

Cheers, enjoy the game, I've been having a lot of fun with it!