One common mistake is that people don't realize they have to parry each individual hit. If an enemy is doing a combo or a multihitting move on you, you'll need to tap towards+weak attack for each hit. Generally, if your timing sucks, you'll want to parry earlier rather than later... this way, you'll just get a block and take no damage (except in rare circumstances). The parry window is pretty generous, moreso on the easier difficulties and it gets stricter on the harder ones. If you parry the attack at just the right time, you'll get a perfect parry which will lead into a crippling counterattack and often an instant Zandatsu opportunity on a lot of enemies.
Alternatively, there's also a dodge move with generous invincibility frames called Offensive Defense. You have to buy it for 2000BP in the Customize Menu. It's a really good tool, and you can choose which direction you wish to dodge. The dodge even incorporates a slash in the middle of it so that you remain offensive. Both the dodge and the parry have their advantages and can/should be used in combination in the middle of fights.
I'm currently playing through the DLC on Very Hard, and after a short while I just couldn't get bothered no-damaging every single fight anymore, so I started going for the no-kill bonuses instead, and it makes a lot of the fights a lot more manageable. I should have done that way earlier. It's a lot more fun to play that way as well, I don't have to restart every 10 seconds anymore.
