Here's the build I've been using on Death March with great success. I don't want to cheese through the difficulty with a heavy-sign build, so the focus of this build is still melee combat. I created the build with leveling in mind, since the early game of Death March is by far much much more difficult than late game. The builds after assume that you're actively collecting Places of Power throughout the playthrough (e.g. by level 10 you should have all the places of power in White Orchard, and by level 18 you should have all the places of power in Velen).
Level 1: Sun and Stars
This skill is simply essential at the start of the game, and will save you enough food to feed the ever hungry Death March. Use this skill for most of your healing. Mediate til daytime when night comes.
Level 4 build
The tooltip doesn't say it, but Focus is actually an absurdly powerful skill. For every adrenaline point, you gain 5% weapon damage and an additive 10% sign intensity. It's also worth noting that at this point School of Griffin is just as useful as School of Cat. I would recommend taking both talents while leveling, and switch to the one you have the best gear for. E.g. I used Cat while I was using Temerian Armor (lvl 4), then switched to Griffin when I got Nilfgaardian Armor (lvl 10).
Level 10 build
The 2 levels in Delusion should allow you to pass most Axii conversation checks in Velen, allowing you to earn quite a bit of bonus exp. As you should realise by now, exp gain from quests vastly outweigh exp from kills, so that big 40 exp a pop for just waving your hands around in conversations is massive.
Level 18 build
Take note of the single point in "Acquired Tolerance". At this stage, a single point here will increase your max Toxicity by anywhere between 50-80. As a result, you will be able to pop 2 potions in a row while a decoction is in effect without start losing health. You will want to swap out "Sun and Stars" with "Acquired Tolerance" prior to boss fights or simply very difficult fights, since without the talent you will not be able to drink ANY potion while a decoction is active without losing health.
Level 30 build (Note: slots 10-12 are not displaying properly on the calculator.)
Slot 10: Sunder Armor
Slot 11: Counter Attack (optional)
Slot 12: Resolve (optional)
One of the disadvantages of fast attacks is the relatively lower damage per hit, which makes heavily armored foes relatively more difficult to deal with. With Sunder Armor unlocked, you can apply the Sunder Armor debuff to an enemy by hitting it once with a heavy attack, then all your subsequent fast attacks will enjoy the damage reduction on that enemy. At this point, you may consider swapping Sun and Star permanently with Acquired Tolerance, since the healing should be negligible now and you should be able to at least afford food now at level 30, I hope.
Final build - around level 35
Slot 10: Sunder Armor
Slot 11: Crippling Strikes
Slot 12: Optional Combat Skill (Resolve is best for boss fights, Deadly Precision or Counterattack are useful for group fights)
Not much explanation needed. Happy hunting!
I will be answering any questions on this reddit thread.