Is it a big guy with a lance? I think you can stick to one side of him and the trick is to stay close circling him with your shield up, close as in like touching him basically, then tap him once or twice when he does the double or triple attack and is recovering
The big pursuer dude with the shield is lije fucking impossible. I died like 30 times then finally someone had laid a summon sign there which was real lucky. The pursuer dude ran at the other guy then I just charged over to the ballistas then shot him 3 times and it was over in like ten seconds. You need help with that one i think