Make a pen
Get a wyvern
Pick up the alpha alla (so far I've had luck with picking the right one up and getting one that's level 130+
Tame it! I recommend raw mutton. Make sure to wait a few seconds in between hits, you may kill them and take awhile to go unoncious between the final hit.
If you get a pack of them they'll bace alpha boost (most likely mate boost) and the bleed out effect.
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.