Can you use game guardian to unlock achievements?
I've never tried, but it I think it might be possible. It's worth noting that I play mostly on my pc (bluestacks emulator), and that game guardian is a mobile memory editor. Game guardian is great for games like this because you can easily search for specific (numerical) values, and also bypass the encryption the game uses.
However, as I'm on PC, I can also use cheat engine which isn't as easy to use to bypass encryption, but is far better for string editing (which I think would be to way to unlock achievements, if it was possible).
I've used cheat engine to "add" cosmetics to my character. It's possible to temporarily unlock items by changing the criteria for it to show up. For example, I can search for the string
ownsComponent("o_Female_VampireInspiredAOutfit_FULL") or
ownsComponent("c_male_SiriusBlackHair_skin"), and change all those strings to
not(ownsComponent("o_Female_VampireInsAOutfit_FULL")) or
not(ownsComponent("c_male_SiriusHair_skin")). By adding not() around it, and deleting 5 letters to keep the string the same length, the client is tricked into temporarily displaying the items as if you own them. The client does reset after you exit and reenter the game, but you keep the items equiped.
In the past, I also tried looking for the seasonal TLSQs. I did find some strings relating to triggering the start of the quests, but I never attempted to edit them as I was less expirenced at the time. It might be possible to "trick" the client in the same way to unlock achievements.