Very easy to tame if you know what your doing. First tame an Ichthyosaur. Then grab a crossbow and some tranq arrows and find one and knock it out. It is not be able to attack if you are on the Ichthy. As you are taming it, be sure to remain on the Ichthy unless feeding narcs, so other creature don’t hurt you.
I dont know why the dododex consider dunkles torpor drain as high, just keep hitting it with tranq just like regular tame, it drops pretty slow compared to other tames that classified as high torpor drain. You didn't even need narcotics to tame it. But maybe it classifies as high torp whenever you miss your tranq arrow, because it is dropping fast.