Only level up speed until you can’t level it up any more or it’s sonic then breed it with one that you only leveled up strength and you might possibly get a snail that can take down Carnos, Dire bears, or Rex’s or not I’ve never tested it :)
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.