A saddled high level equus can be a huge help in knocking one out, as long as it has enough melee and stamina. You can't outrun the Allo, so let it catch up to you and kick like there's no tomorrow! Your Stamina will probably go down much faster than your health. Keep kicking until it runs away, let your equus rest, and chase after it. KO it with darts or tranq arrows, which has the added benefit of lowering the Allo's chance of dying. This strategy works even for high levels.
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