There are now 10 Guardians available in the game, though you only need five for the achievement.
- Vampire Bat
- Fractured Ward
- Ram
- Snake
- Exemplar
- Watcher
- Tiger
- Owl
- Astral Tiki
- Lion
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. ***