My Geralt lives by the Golden Rule, "do unto others before they do you." In Dark mode, this is the corollary of "don't get killed, witcher." Practically, this means spending talents on skills that do crippling damage from range or in the initial attack, at the expense of defensive skills and Vitality bonuses. It calls for advancing to the top of one skill tree (which one? your choice) as early as possible and promoting Adrenaline generation. It calls for maximizing the ability to deliver critical effects and one-hit kills. So these are my character development preferences:
Early in the game, get the third Vigor point in the Training tree. Later, use surplus talents to acquire Vigor points.
As soon as you can, build to the top of your chosen specialty. Once you're there, emphasize anything that produces Adrenaline.
Even if you do not emphasize swordsmanship, build out Riposte. Even if you do not emphasize magic, build out Aard. Even if you do not emphasize alchemy, build out Alchemist (bombs and traps).
Use mutagens when you get good ones. Critical Effects, Enhancement, Power, and Madness if you ever find one, are good.