I'm having an hard time hunting the king of the crypt, Ozzrel.
Geralt is at level 9, has spread his steel sword with Necrophage oil and prepared himself for combat meditating and drinking Swallow and Willow.
He is level 2 in all three combat styles, and has got level 1 Aard and Igni signs.
Upon engaging Ozzrel himself, Geralt has used the Circle of Elements (+ 50% Igni) and the Place of Power nearby (+100% all signs).

Geralt then engage Ozzrel casting Igni sign whenever possible, then going melee with Fast style combos (to cause Pain) followed by a a Strong style combo (to deliver real damage), dodging enemy attacks when possible (when I actually see the attack animation, because without it I have no clue when to start dodging vs start attacking again to possibly prevent the enemy to attack Geralt) and generally running around for a while avoiding Ozzrel if vitality is low.

The problem is that even if Geralt deal large amount of Damage to Ozzrel, he eventually dies because Ozzrel can kill him with just a couple of hits...

Any advice form fellow witchers?

-----
I'm playing at Medium Difficulty, with Flash Mod installed (Harder Dificulty chosen).
-----

Not sure how Flash Mod affects this quest, but signs are very effective in the base game.

Initially, Aard is the best way to deal with necrophages. Stun them and one-hit them. Ozzrel is susceptible to this.

Later, Incineration is a big help in dealing with difficult monsters in general. You need the second level of Igni to get the Incineration skill.

If you are taking on Ozzrel with just a sword, go with Strong attacks only. Fast attacks won't hurt him enough. I favor the sequence Igni, Strong combination through the final attack, back off, repeat as needed.

I second those suggestions. Necrophages are immune to pain, so use Strong style only, and keep on him, don't try to dodge and run around him - if you retreat, retreat for real, far out of his reach, but then close back in. Willow can be replaced with Tawny Owl to allow you to cast your Igni sign more often.

Also I think Flash mod overrides the normal difficulty settings, so if you chose "Harder" it will be just as difficult no matter which difficulty you pick in-game.

Thank you for your suggestions, fellow witchers, finally Ozzrel is no more.
Geralt fought like you said (Swallow + Willow + Tawny Owl) spamming Igni and going melee with Strong Style with Necrophage Oil, then retreat & recover (if damaged) or engage Ozzrel again.

I've lured him near the Crypt entrance so I could see him (no Cat potion).
Ozzrel fought for a little while, then he turns his back away and started to walk back to his chamber.
That was the perfect time to strike :)

------
Geralt did not use the powerful Aard sign 'cause with the Flash Mod, among other things:
- all bosses (except the Beast) are immune to stun
- all bosses (except the Beast) are also immune to critical hit
------