There are now 10 Guardians available in the game, though you only need five for the achievement.
- Vampire Bat
- Fractured Ward
- Astral Tiki
You can acquire Guardians by buying packs in the Emporium. The cheapest pack is the Common Pack (500 gems). Just keep buying it until you get five different Guardians.
***After getting the final Guardian, the achievement didn't unlock for me. I had to buy one more pack and then it popped. ***