Get someone who knows what they're doing to show you some tips and tell you what to improve, as Droid and did for me. It really helps.
Spar opponents better than you - You learn more from losing. Make a practice account if you're worried about your score.
Enter a sparring guild, when you guild spar you can learn lots of techniques and put them to use in singles spar.
Finally, practice makes perfect. Nothing helps you improve more than dedication and practice.
As far as techniques go, I'll list what I do in different scenarios.
Your opponent has good wifi?
Spar normally, hit once then move away to a different location. Try not to be predictable and predict where you think they will go and swing your sword there.
Bad wifi
If your opponent is laggy and stops moving but cannot be hit, hit everywhere in the arena. Once their wifi catches up, everywhere you've swung your sword will appear for them. If you've swung your sword everywhere you're guaranteed to get a hit.
Delay
I find fighting delay tricky, try and stick close to a wall and swing your sword a lot. Predict where they'll go and swing there, make sure to keep them at bay as on your screen they might look like they're 3 tiles away from you but in reality they're right next to you.