Master blocks. The key to blocking is that you can always block, even if you're in the middle of a swing. Also, you REALLY need a high athletics skill.
Contrary to what others are suggesting, don't go for the backpedaling tactic. Mix it up a bit. If there are 3 or 4 dark knights rush at them to break their file and hit them where it counts. If you backpedal, they rush at you. If you rush at them, they backpedal. See how this works? If you strafe around a bit and one of them gets seperated from the file, charge him and take him down then spin around to the group again. I can take about six-seven dark knights with my two handed axe and rarely get hit. After practice, the only trouble is when two enemies swing at you in different directions at the same time. That sucks hard.
Use throwing weapons. Master them. If 9 dark knights charge at you, you'll want to be able to hit at least three of them in the head before they get close. If you can't hit a guy in head from 3.0 range, practice.
Unfortunately my level 28 guy's savefile is all bugged up.
Hopefully I can get back into it after a patch or so.