Playing on Death March, toughest fights so far for me were the large Wolf packs containing what 7 Wolfs and 1 Warg? Just hit Velen, haven't gotten into a fight there yet.
My biggest enemy so far has been myself when getting overconfident and getting killed while going all out on an enemy.
Best advice for the higher difficulties I can give is go at it slow, take your time, study your enemies movements and DON'T get greedy and loose focus, even when it's only 1 mob left out of a pack.
And I love the fact, that sometimes I had to retreat from a fight, because the enemies are too tough. It makes you feel that you are vulnerable, and not a terminator from the future.
I always try to use the environment against my enemies. Damn, you can even use your enemies against your enemies lol
Like when you find a "pack of humans, and there is one of them with a bow. I always try to make the other guys go between me and the bow guy, so he can kill them for me
I don't know if I gain the exp for that, but it's pretty fun though