I use to go to the beach, swim out a little bit until it agro. Then swim to land again and the cnidarias are following me to land. When it I am standing on land it stops. Then I can easily kill it without taking any damage.
For the people complaining about the iguanadon being one of the highest rating, its because of aberration. The iguanadon harvests a bunch from the red hallucinagenic mushrooms that are unique to aberration.