Note: Some SC terminology will be used in this solution.
A: Horizontal Attack (X button)
B: Vertical Attack (Y button)
K: Kick (B button)
G: Guard (A button)
BE: Brave Edge
Fighting is more mental than you might think, and mixing up your high-low game is a great way to mess with your opponent's head. Keep this in mind while playing your Ranked Matches.
A great move to open a round with is Forward, Foward+A, B (or Down+B). The initial slash will immediately close the gap between you and your opponent, hits mid-height to stop low blockers, and has a wide enough swing to stop opponents from starting the round with a sidestep. The B button at the end will cause you to fire a crossbow bolt shortly after the slash, and pressing Down+B at the end will make you fire the bolt low to get under the opponent's block.
A simple move with a surprising amount of power is Forward, Forward+B, B. Just one of these attacks will knock out about 20% of the opponent's HP, and it hits at medium height so it'll connect if your opponent blocks low. It's relatively quick, and comprises of two hits, so players unfamiliar with Ezio's moves may stop blocking after the first hit. However, while it's fast and powerful, it's also very linear, meaning the opponent may sidestep if they start to catch on. If they begin to navigate around this attack, feel free to throw in the Forward, Forward+A attack to discourage them.
If your opponent is blocking high, using Down/Back+A will result in a low, sweeping attack with the crossbow. If you're fortunate enough to land it on a counterattack, you'll automatically shoot a bolt into the opponent for extra damage. However, it's rather slow, so don't depend on it too much, but it is a nice move to use if your opponent doesn't watch his feet too often.
Tossing an occasional throw in is a good way to deal some unblockable damage, especially if your opponent is constantly defending himself. However, if you rely on throws too much, your opponent may start to duck more often to avoid your grabs. This is a great time to capitalize on the Forward, Forward+B, B attack and its mid-height damage again.
Ezio doesn't have much for Ring Outs, but his low crossbow can knock the opponent's feet out from under them and off the stage if their back is to the edge. A great way to ensure they go over is to use the Forward, Forward+A, Down+B attack. The first hit will get them blocking high, and the second hit will bring them off their feet and over the edge.
When your Critical Gauge is filling up, don't be so eager to use it on your Critical Edge attack. The Forward, Forward+B, B attack has a BE version that increases its power and adds another hit to the attack by pressing RT immediately after the second time you hit B. If your opponent has gotten used to the normal version of this move, use the BE version and they may stop blocking after the second attack. Not to mention, the added damage is always a nice bonus, plus it only costs half a bar of your Critical Gauge, so your opponent might not expect such a powerful attack early on in the match.
This achievement is best done early on, while you're still low-ranked and fighting against other low-ranking players. As such, some low-ranked Ezio players may be too dependent on the crossbow attacks. Just remember that all crossbow bolts can only be fired at high and low heights, meaning that all damage can be prevented just by blocking low. If they're so dependent on the crossbow, it's likely that they may be unskilled in close combat, so attempt to close the gap between you in that situation.
If you're still feeling unsure, try out the moves in Training Mode, or go into some Player Match lobbies to get some practice against actual players while seeing how the online lag can affect your timing. Experience is the best teacher, and if lots of players fall for certain attacks in Player Matches, a good number of them will probably fall for them in low-level Ranked Matches.