Allos' are like land sharks. They can see you from a mile away, travel in packs, and are extremely fast. Chances are if you can see it, it's too late for you if you're on foot.
Best chance for survival is find another Dino and run beside it to try and get agro off yourself
if u are going for an allosaures tame the easiest thing you can do is lead one into a taming pen and then kill the others outside that way they do not have the alpha pack boost affect and you can easily and safely knock it out