No pre-quests
For me this is the easiest pet-quest away from North Island
1st:
Go to Maya (you can use undergound or vehicles)
Enter Mayan Tribe and go to the 1st house at bottom.
You will see a cut-scene of guards taking shasha to the "Deity Pool"
Then get out, go left and enter the well you will see.
2nd:
When you are inside go upwards and right you will se locked door and a statue. Push the statue to get in.Then just follow the path to the altar, there you will see shasha lying on the floor.
3rd:
Talk to her till you get to the battle:
ET
water lv 118
12000 hp
Reward:
Shasha joins your team