Hello,
I've started ninja recently as Berserker didn't fit my taste and I've been trying to find some guides for this class but I can't really find anything
Does this mean that ninja is bad and noone plays it?

I've tried Atoraxx dungeon solo on Ninja and it's nightmare vs Berserker on 252 AP/310 DP or is it really that much harder vs Season servers?
I feel like I don't deal damage to anything

Well you've gone from the most braindead class to arguebly one of the hardest.

There arent any ninja's out there cause the skill ceiling is very high, not that the class is shit.
Ninja is the best if mastered and it takes a long time to master it fully.

Here are some videos to help. Just letting you know if you do decide to keep being a ninja then the road is going to be very long and hard but very rewarding.

Beginner Succ PvP/PvE Guide
https://www.youtube.com/watch?v=twzzs9v6wSk&t=224s&pp=ygUTbmluamEgZ3VpZGUgYmRvIFB2RQ%3D%3D

Beginner Awake PvE/PvP Guide 
https://www.youtube.com/watch?v=JMWKpJaeMN8&pp=ygUTbmluamEgZ3VpZGUgYmRvIFB2RQ%3D%3D

Hope these help.