Main game no DLC.
After tournament (quest with dead son of blacksmith and knight in black armor) I ride a horse to Samopesh, but before first bridge there's a trap with bandit with poisoned sword. I used 5 different potions to make my chara better but still this battle is impossible to win. I even put two arrows in him and he's still in god-mode. (one arrow in his head)
How to pass it?

Well, had my own issues with him earlier, so I can understand the frustration.
Since he is well-armored, using a blunt weapon would be preferable in most cases (or a really good piercing weapon). It also helped me to approach the fight very defensively, letting him attack, block and use the small window of opportunity to atttack him, since him just getting one hit in early with the strong poison he uses will likely kill you over time even if he does not managed to hit you again. And since he is good with blocking himself, standard attacks will just get you killed.
I also had some poison left over from an earlier main quest an had little moral reservations using that

Yes he is quite hard to beat and in full plate if I recall, you might want to use a poison yourself on him like a dollmaker potion and blunt weapons would be best against his armor type. If I recall correctly he has a closed face plate helmet so head shots with arrows won't do much damage.

Rest assured he is beatable just hard to beat.