I think what I dislike most, which often annoys me on hard settings, is that I die in 3 hits and enemies die in 30.
I'm not playing at the hardest difficulty (DM!). Only one below (BaBB!). I did play TW1 on the hardest difficulty. (The boss at the end of Act I is really tough. I had to edit my savefile to make a change to my talent-spec. And then I could finally do it).
I think when you play at the highest level, you need to make use of all the tools there are. That means:
1) Get an entry in the Bestiary. I think they are not just tips for the player. But they could actually enhance the rpg-stats of Geralt as well. (how hard you hit, how hard you get hit). Not 100% sure. But that's how it works in TW1 for sure. In TW1 I would actually spend all my money on buying the books that have the right entries. (I did use the wiki to find out which book I needed for an upcoming fight).
2) Use the correct oil. Only 10% or 20% or so increased damage. But that's a pretty big buff, imho.
3) Use aggressive signs. Don't use Quen. Just try to make sure you don't get hit. And use Igni or Aard. Aard was very good in TW1 and TW2. Knock-downs meant certain death for the opponents. I haven't seen Aard be very good yet in TW3. However, Igni is awesome against anything that has a fur. They just burn. Igni is also good against any water-related enemies (drowners, water hag, etc). Make sure you make the best use of your signs. For Igni and Aard, try to use them at a time where you can hit more than 1 opponent (they are frontal cone attacks, so a big potential max total damage if you fight multiple enemies).
4) It seems that when enemies roam in packs, you can often pull a single one with your crossbow. Shoot one, watch it come to you. Back off a little, so you will be out of range of the attention of the rest of the pack. Fight the single enemy alone. Make sure you don't keep jumping forward towards the pack (Geralt likes to do that). Kill one. Repeat.
5) Use your potions. Health regen. But also the attackpower ones. In TW1 and TW2 potions were very powerful. In the books, Geralt takes potions before every fight. So even if you want to role-play, taking a lot of potions is fine.
6) It seems you can eat and regen health while in combat. (Lots of games don't allow that). I always have a large stack of meat on me. And keep eating while fighting. If my health drops really low, I take a Swallow. So far that's been enough to kill boss-fights.
7) And obviously, keep your gear up to date. Like in any game. When I saw the recipes for the Viper swords, I made sure I made them (both) before I fought the griffin. I just got a hint for more gear, but the quests are lvl17 and lvl24. I play to try the lvl17 one asap when I hit lvl12.
I don't care much for real challenging fighting in games. But I don't want it to be a complete walkover too. I enjoy the fact that I can beat monsters because my Geralt is very good (rpg stat-wise). Not the fact that I am very good. So I play on Blood and Broken Bones right now. I'm slow. And just level 7 or so. But already many fights are pretty easy. Maybe I should restart and play Death March.
Hope this helps.