From what I can gather, Sawyer knew that Hawlucha was best at fighting opponents who, for lack of a better term, moved around. So he could trip them and stuff. So he chose Slaking who doesn't move at all. He then took advantage of Slaking's bulkiness, as well as Slack Off recovery, until Hawlucha just began attacking blindly — which was the pefect setup to KO it with Counter.
Now, Clemont said that Slack Off didn't heal all of Slaking's health, hence why Talonflame was able to take it out so quickly. However, Slaking's purpose was never to beat Talonflame in the first place, it was there to beat Hawlucha.
Was that more clear?