If you are at a higher level and you have a longneck rifle expecially one with more than 100% damage use it from a little ways away and you can kill them easily. Then walk up to them and harvest them.
If you dont have the longneck rifle i suggest dont go near them, but if you are a savage and want to kill one take your pike or spear hit it then run. Then turn around with any ranged weapon and just keep shooting. If you dont have a ranged weapon i suggest you not trying to kill one because a few hits and you are going to die.