Gems of War

This achievement is unmissable. I unlocked it when I was around lvl. 14.
The different kingdoms require you to have a certain level, so you actually have to wait till you are lvl. 14 (EDIT: lvl. 12 seems to be the accurate lvl) too, to unlock this.

In adition to the level you need to have a certain amaount of gold. the costs to unlock kingdoms vary. It's relatively cheap to unlock "Adana", but it takes some gold to get kingdoms like "Whitehelm" or "Divinion Fields" ( 1,250 and 1,500 gold pieces).

If you struggle with your gold use the daily log in bonus. Just make sure to at least start the game once each day. This will give you higher gold bonuses each day.

Here are the five kingdoms I unlocked to get this achievement at lvl. 14:

Broken Spire
Adana
Karakoth
Zhul'Kari
Whitehelm

Instead of Whitehelm, you could also unlock Divinion Fields as your 5th kingdom but it will cost you more money than Whitehelm, so I would save it for later.

EDIT: in the comments the following hint was posted: " I suggest doing Arena. You buy yourself in for 1000 gold and if you win all 8 rounds you'll end up with 2 keys, a couple of thousand gold, 500-600 souls and 10 trophies."

Hope this helps!
Any improvements? Please let me know in the comments below!