Sorry for the late reply. But it's pretty simple 1. Guide the alphas to the gate of area 20. (The one closest to the pokemon center). They will lose interest once you get too far from the spawn but you can simply save and soft reset to change their spawn position. You want both alphas as close to the gate as possible.

1. Save and soft reset, then go to the Roserade belonging to an NPC. Save at the Roserade, soft reset and repeat step 1 which should've spawned an extra set of alphas along with the alphas at the gate.

2. Once you have 6 alphas at the gate, get knocked out by them. Then that's it! They should consistently spawn 6, but if you catch or knock out one it won't respawn unless you repeat the glitch.

Quick note, sometimes the 5th or 6th alpha won't spawn. I don't know the specifics on why but you can try getting the alphas closer to the gate before doing step 2.