Gems of War

You have two options right now: Wait for troop releases that makes this achievement way less "grindy" or you can use the hourglass gems that will show up during each fight with the current campaign running.

Example for troops in the future:
Nov. 6 - Tempurath
Trait #3: Temporal > 50% chance to convert a Yellow Gem to an Hourglass Gem when my turn begins.
Nov. 13 - Daughter of Time
Spell: Create 5 Hourglass Gems and 5 Freeze Gems. Then gain an extra turn.
+ Trait #3: Temporal > 50% chance to convert a Yellow Gem to an Hourglass Gem when my turn begins.
Nov. 31 - The Hermit
Spell: Create 2 Hourglass Gems. There is a 7% chance of an extra turn for each Yellow Gem on the Board.

So the easy method is to wait for the sixth November to use a team with a couple of Tempurath to trigger the 50% chance as often as you can. Good to know: Destroying hourglass gems with Thrall or similar troops count also as match.

However, if you are less patient, you could grind this out with a team like in explore battles:
Banner of Sin + 3 Nysha medals for more destroyed gems
Thrall
Marilith
Thrall
High Priestress Ch'azka

Your goal here is to simply destroy the whole board to let hourglass gems spawn with a rare chance. The Priestress have a trait that creates a lightstorm each turn. I'm not 100% of more yellow spawned gems means also more potential hourglass gems, but it's easier to match them with a board full of yellow anyway. Progess is added at the end of the match and you probably need more or less 1 hour of grinding, depending on RNG and enemy moves.

edit: "Was I That Bad" pointed out the Stormcaller class also have a trait for lightstorms. So if you miss the Priestress troop, you can use this class instead.