On a 135, 13 crossbow shots DO have a chance of killing it. Like so Dododex can see.

They are skittish but not anywhere near faster than humans. They have low torpor and health, so tranq darts are recommended. Tamed like any carnivore, kibble is Archeopteryx. Can be found anywhere in and around the swamps.

Useful to supply oxygen to divers who do not have scuba gear at hand but very weak in battle.

Diplocaulus are constantly picked on by swamp carnivores, even the sluggish Dilos, add on the fact that they are so painfully slow and it results in it constantly kicking the bucket.
For taming either take some strong tames to kill all swamp carnivores or, and this is worth noting, use a pteranodon/Tapejara to hook it out the swamp for easy pen taming