I started on the second hardest difficulty but it was proving to be a little too easy. ( I Love a challenge )
Don't get me wrong, I died a fair amount in the beginning and certain fights required many attempts. (Arch Griffons and the General at the end)
I switched to Death March at level 15 roughly.
Basically the things to avoid are standing in the middle of packs,
going toe to toe with the Pike wielding NPC's and generally standing in front
of any big mobs.
Movement is key. I cannot stress this enough. ( I assume you know this)
Quen is your friend, eat breathe and sleep Quen.
As for gear, witcher gear all the way. Its the most balanced and its ahead of other armour at the same level. At the end I had all 3 sets of Master crafted.
I forgot the name but the Heal potion (2 uses) is very helpful.
I used Fast attack 95% of the time. (Whirlwind of Death)
Use the talents that generate adrenaline when using signs etc.
Use gear that boosts Adrenaline gain, the more adrenaline, the more you
can use Quen.
Again this was just my play style, each to his own
Meh. I played on maximum easy with the best armour and not many signs, just whacking. Otherwise too much effort.