This game is easier with friends. Find a place where there no other mammoths and start loading it up with tranquil darts or build a wooden tower out of ladders pillars and ceilings and shoot from there it should be easier that way. Once down surround in spikes because there's a chance that something with come to kill it.Good Luck!
get on a nearby hill or rock and kill ones you don't want as they usually spawn in 2s or 3s (recommend an argentavis). then start shooting the mammoth with tranquillisers. when it torpor runs finish it off, they are very slow so very easy to hit. (protect from dire wolves, yutiranus, etc....)