Sun and Stars ability mentioned above is useless when you are farther into the game, because you have so much health that it takes a long time to heal. I have not wasted points on it in my death march game.
Against strong foes I use a combination of Yrden, Axii and Igni. For Yrden, put some points into the ability that makes Yrden deal damage, use Axii to stun enemies while they are in the Yrden area and then set them on fire and start whacking away at them. Yrden is also useful against many other foes like wraiths and because it slows enemies reactions down so that you can hit them better before they can turn around to attack you. I also use Quen, but don't have a lot of points in it, still it is very useful against enemies that deal a lot of damage... keep casting it every time it breaks. The burning ability for Igni is very useful, it makes many foes stop attacking for a second so that you can attack them with your sword.
Above all, learn to use the evasion abilities... rolling especially. Getting hit by most things will take a significant chunk of your HP even if you are ridiculously overleveled for them. Always keep moving fast to avoid getting hit by projectiles. At level 36, with mastercrafted griffin armor, an arrow from a bandit still takes a significant chunk of my health.
Learn to anticipate the enemy melee attacks and roll away just before they can hit you. And if you are not running away at the moment, make sure that Geralt always has the sword in defensive stance, this will mitigate many attacks.
Use all the potions, bombs and oils that you can, and I have found the ability that restores health by up to 25% instantly when using a potion very useful. Keep your armor and weapons updated with your levels, and use runes with them. Put abilities with the same color together in ability slots and upgrade your mutagens to use the best ones that you can.
In Death March, you have to use a lot more tactics to win battles, but that is how I like it.