1. Use cursed weapon, I managed to wave 13 without any trouble.
2. Use poisoned weapon, I don't like this tactic, because after few waves, you can't see ****
3. Use instant charging sword, I recommended use the sword from Roman pack with combo multiplier, keep charging and no one can touch you.
4. Take out archers first, don't worry about big guys, there're really easy to kill. And don't kill animals if they're not attacking you, they'll keep other enemies busy.
Here's my video, not perfect, but you could do better than that:
1. you didnt use a cursed weapon in this video and suggesting it isnt really helpfull as archers can basically one-shot you starting at around waves 10-12 if you only have 1/3 of your health (maybe even earlier) so while the damage output would be nice, cursed weapons are pretty much worthless on nightmare due to archer damage scaling
2. poison could be a valid strategy, cant comment on it too much, need to test this myself but i doubt its going to be the best way to go
3. solid advice, and in addition to that use a weapon with berserker-mode (kopheshs for example) to really up the damage-output as you are almost in god mode during berserker (if your instant-charge weapon has adrenaline generation, that would be even better for that tactic)
4. definitely!!! archers are the one and only big problem on nightmare difficulty (if you are decent and dodgeing and parrying)
5. when a new wave starts, stay at the edge of the arena, where they cant hit (or shoot) you in the back right away and keep your shield up until you have located the archers