If I recall correctly, you are playing saved games (not real-time) against opponents with a similar ranking to yours. These are games that were played by real people.
So if your ranking is 500, your opponent will rank somethere between 475 and 525 for example.
I thought that all you had to do to "win" was to beat your opponent (better total numerical score), but perhaps that is not the case.
Also, it is not just the numerical score you see at the end of the game / time limit -- all the bonus tokens (silver coins) get added to the total point score, so you want to collect a lot in challenge mode.