What I do to kill these things is just get it to the shoreline, then it can't hurt you while you stand in front of it and beat it to death with a bat. Literally the second species I killed after some dodos.
If a cnidaria killed my spino, boy i would be mad. i lost my sarco to cnidarias, a scare with my icthy with cnidarias, and almost got my megalceros killed by some. ( he is dead now D:) I kill any of these that i can. The dossier says that they are passive, but aggresive when approched, NO! as soon as i step in the water these attack, and they litterally don't move at all, they just sit there and chill until you or a tame steps in the water. one like=one prayer for my deceased sarco, Gatorade (you can find his story in sarco) and one member of the cnidaria hate club.