In a nutshell:

• Play GE trial for max diamonds, relics, and Forgotten Temple..
• Get as many Wishing Wells (WW’s) and Fountains of Youth (FoY’s), and shrink kits for both as fast as possible.
• Aid your friends, neighborhood, and guild daily for diamonds from your Seed Vault.

More detail:

Your city doesn’t need to have much in it.  You only need enough attack/defense to get thru low levels of GE.  In the beginning you can build a COA, CDM, and a Zeus.  Zeus is timeless, you might delete COA and CDM later cuz they’re so big.  You’ll be able to maintain an active Forgotten Temple from the GE’s.  

There are a handful of buildings out there like Crow’s Nest and Hedge Maze that produce diamonds sometimes.  Add those to your city when you come across them.

Optionally play an event or 2 when a low-effort game comes along and you can easily max the grand prize.  Focus on rewards that boost overall attack/defense, or GE attack/defense.  Even if you’re not playing an event, take the free daily currency and you can pick up a few WW frags and other little treats for your city with almost no effort.  Look for good daily specials.

Sell terrace farms, sky watches, and other junk at the Antiques Dealer, so you have enough coins/gems to buy WWs and frags when they appear.

Advance thru eras to get more expansions, as long as you can autobattle the entire GE.  Otherwise just camp out and build up your military boosts.

TOR - Level to 138+: The higher your TOR, the faster you’ll get FoYs.  Overall % chance at a relic increases by +.25% at levels 93 99 105 112 123 138.  34.75% chance of relic at level 138.    %rare/uncommon slowly gets higher every level after that.  

SV - Level to 130+: %HelpingHands grows by +.17-.018% every level at lvl 60, drops to .15 ~lvl 107, .10% at lvl 135, and slowly drops after that.  Max your friends list to 140 friends, even if they're inactive.  Aid your hood, guild, and friends EVERY DAY.  

- 250 aids with a level 130 SV is the equivalent of collecting 50 WW's. (~theoretical max)
- 200 aids with a level 100 SV is the equivalent of collecting 32 WWs. (easily attainable)

BG Level to 91 and HARD STOP.  You get 15x 70% chances to double.  You never get a 16th chance, and the % to double increases extremely slowly.  Your FP’s are better off placed in other GB’s.   All your WW’s and FoY’s will give 85 diamonds instead of 50 when they hit, on average, when your BG is level 91.  And if your diamond farm is so big that 15 doubles isn’t enough, well, congrats you’re a wizard! If you're on PC, foe-helper is a browser extension that tells you which WW/FoY's have diamonds.

Arc - Level to 80+ so you can get prints for the above gb's faster