If you are looking for a high level one they spawn in caves on SE. once you find the one you want grapple it and fly home to do an easy tame. You can also do this with any panther ones that spawn else where.
Achatina's are by far the hardest animals to escape on the island. Their insane movement speed is devastating to you and your dinos. These snails are also very powerful and will kill your gigas within, at least, 24 hours! The best thing to do when you see them is bola them and run.