ZEN

Astral Diamonds can also be exchanged for Zen in the Astral Diamonds Exchange. To buy Zen, you must input the highest price of 750 Astral Diamonds for 1 Zen (otherwise it just won't be processed ever because of the demand being too big), and wait. And Wait. And Wait. And if you're playing on PC : wait, wait, wait, wait, wait, wait, wait, and wait a bit more. It actually takes more than 6 months to exchange AD to Zen on PC. It takes about 2 months and a half on PS4, and a bit less on Xbox. Yes, it takes a while, but you'll be able to get everything for A LOT cheaper : wards, choice packs for weapon/armor enchantments, upgrade tokens... Wait for the next big sale (Black Friday / New Year / Anniversary in June) and get yourself VIP rank 12 (very worth it) and anything you want for basically half the price you'd find it on the Auction House. Considering it takes at least 2 years to max out a character, don't make your impatience make it 4 : don't buy anything on the Auction House, unless there's no other way, and exchange all of your AD to Zen.

Beware : you can only put 5 offers for a maximum of 5000 Zen (3.75M AD) each. So do not put small offers or you'll have to remove them to put bigger ones... and it's first come first served, removing your offer means going back at the end of the line.