You would want to get onto a flying creature and fly onto a big buolder or something like that we’re it can not get you. Use at least a crossbow with tranq arrows. Then you can shoot it down. Make sure you have enough arrows. Once you got it asleep, feed it with meat. Then you should have a Allosaurus! And if you want to ride it, you need to unlock it’s saddle at level 67. Now you can ride your Allo!