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.
You mostly find them in caves, near mountains and also near swamp regions. They can’t really run away from you but taking them isn’t that easy either and might take a little more resources than you think at first but if you are far and rather prepared in the game this should be no problem. If you take them and put them on „wandering“ they will „snail“ around and slowly produce up to 100 Achatina Paste and 3-5 Organic Polymer if you leave them alone for a while. A simple short exploration is enough for these bad boys to generate a lot of Paste.