A Supreme victory is one in which you defeat your opponent twice (by reducing his "life" meter) without having been defeated once yourself. The first time, your opponent will fall, knocked out, and then recover for another go. The second time, the match is over and you are awarded with the victory screen.
This can be accomplished in a Ranked match, or even a friendly match with a boosting partner, so long as the game is an online match (no local PvP). If doing it against an unknown opponent, make use of your blocks to reduce the amount of damage you take, and go for combos that you are familiar with. If doing this against a boosting partner, he should simply not attack you so that you are guaranteed the Supreme victory.
You must have Glacius unlocked for play in order to earn this Achievement. You could purchase him using real money, have had him gifted to you prior to launch of the game by Microsoft, or you could wait for him to come around in the free-to-play rotation (as of this writing, Jago has had his turn, and now Sabrewulf is free).