This one stumped me for a little bit. I got a couple of fullcombo games and it was only awarding me gold medals. Googling it turned up nothing because nobody plays Wii Sports Club apparently. Anyway, I got the idea to try other punches and that was it hooks and uppercuts are worth more points.
I wasn't too brave with the uppers because the cooldown takes longer. I pretty much only used them in the beginning when it's slow. Luckily that is enough to get a platinum.
Wii Sports Club 100% Playlist:
• Wii Sports Club (100%)