Best way to earn XP?
Play the game, do collections and as many moments as you can.
Showdown. About to non prestige levels per full showdown
If you haven’t done stage 1 team affinities I’d say so showdowns for each division until you get some free diamond FOTF cards. Most of it’s on veteran now so it’s super easy and fast to do. Gets you 24 TA points toward whatever division you complete + fast stubs and xp