Gems of War

This is located in the PvP section. Inside the card restricted regions, there are citadels to attack. Two items on this one. 1. You have a limited amount of sigils so this is going to be 10 guardians over time. You can purchase more sigils per day (so says the devs), but the only bundle I could find in the shop wanted real money. I'm hoping they use in game currency in the future. Also (based on comments below), do not confuse Monolith sigils with Citadel sigils. Buying Monolith sigils will not do anything for this achievement. 2. When choosing a card restricted region, choose one where you can field a good PvP team. Please note: an eligible citadel for attack will have a crossed sword icon on it. Also worth noting, if you go into your Alliance tab in PvP, the Overview shows the schedule of citadels available for attack.

To get to a guardian, you must first beat four PvP battles. On the next battle you will be offered a Citadel Guardian battle. The first one starts at Level 100. Each guardian battle after that goes up by 50 levels.

This one may be a challenge for newer players. It takes time to field decent PvP teams, but then for the guardians, that same team will also have to be able to punch through some challenging enemies. My first guardian had both 75% spell and skull resistance. For the seasoned player, it will just be adding yet another daily task until it is completed.

***Addition***
One commenter has noted if you fail the guardian fight, you do not get a retry. You must beat four more PvP battles. So beware!