Old thread is old, but it bears repeating.
If you as a lefty are not winning at least 50% of your engagements with relentless aforementioned lefty-chops, then you are not doing your job as a lefty sword & boarder. The other 50% of your victories should come from the fact that your opponent should be expecting a lefty-chop 100% of the time; so you feint a chop and juke 'em in their shield side shoulder or kidney.
AS HARD AS YOU CAN.*
I promise, if you can understand those two simple rules, you will pwn the world. When you go up against medium to high skilled opponents your left handedness isn't really an advantage anymore. If there aren't any skilled fighters in your area, try to spar with another lefty, or switch hands for a few months.
Also, don't let people know you're left handed until it's too late. Then it's LEFTY CHOP time. Control the speed of the fight, strike first (generally), strike to kill. Run with the left flank, we have the most fun.
These are really crude instructions and obviously, you aren't going to reach the top with only two moves in your vocabulary, but you would be surprised at how effective those two moves can be if you practice them.
*
Well... maybe not that hard.