Make sure no other Castoroides are around then bola it and shoot it with tranqs until it knocks out. Make sure you bring loads of bio toxin, because their torpor drains fast. I used this method and tamed one at level 42
The way to tame these guys is to run up to one of their dams and then open it and then immediately close it (you don’t have to take anything from it, opening the dam will immediately aggravate all and any nearby beavers). Once this step has been done, you must bola the beaver (assuming only one tries to fight). I actually made a hearty mistake and bolaed the first one that came whereas I should’ve killed the second first (I was making a video, the second one got killed). Once the beaver is bolaed and separated from its mates, go behind it where it can’t hit you and hit it with a bat until you knock it out, but before doing any of these either kill jellyfish for bio-toxin for torpor (the beaver’s torpor will drain quickly) or farm a bunch of narcoberries in preparation. These guys are herbivores so you should also farm other berries besides narcoberries, or gather from the nearby forests. Keep in mind that beavers EXTREMELY slow eaters when knocked out, so patience is your weapon here.
If you wanna tame one. Be sure to have citronal and narcotics (LOTS). Probably 120 or more for a level 100 and above. Though if you wanna tame in 40 mins and use 70 narcotics, be happy if you've got 10x soothing balm.😄
I found a level 16 for my pursuit to tame one. 2 Galli kibble and 10 (probably overkill) did the trick, took 2-3 minutes. 2-3 tranq darts will knock it out easily, just wait a second or two inbetween shots. Best wood gatherer, do not listen to anyone else. Also I've heard it can be a mobile smithy.