For critical hits, I used a hammer. I also attacked players that were around my level. That way they wouldn't lose all their life points with the first blow.
At the time, I was at leavel 13 and I was attacking level 11 and above.
Critical hits is a thing of chance so the more hits you can land, the higher the chances of you landing a critical.
- Use a hammer
- Attack players below and around your level
This is what worked for me.
(Also my strength is at over 5000. That might have made a huge impact, maybe)