I highly recommend the group selection video.
First key moment in Soulkey's demise is he picks Barracks over Sharp for some inexplicable reason knowing full well Barracks is more dangerous to him. 2nd Effort makes a brilliant pitch to Barracks promising him he will pick a Zerg and effectively sabotage the group for Soulkey.
Soulkey makes a counter-offer which is refused by Effort.
The key was always to double-Zerg SK's group but there was never enough qualified Zergs to even make that happen.