Try to make them fall into a ravine. Flyers help with this tactic. Or better, look for some that spawned or fell into one. There’s a ravine on my SP game where there are around 11 allos that fell into it. After getting them into a ravine, kill off all but the one you’d like to tame. Proceed in taming as usual.
It will usually be very difficult to separate one from the pack. I suggest trapping all of them (usually 3 in a pack) in a behemoth gate or ramp style trap and kill the ones you dont want before taming the one you do want.