The Spider Bite is a onehanded weapon, so that with another weapon in the other hand (at that time I had a very similar albeit a little bit weaker secondary weapon with toxicity attrib too) should be the choice, especially as assassins don't really work with strength but agility.
But if you want real help your skills list would also be necessary just to get an idea where's your problem. Difficulty setting can give a better general idea too.
Thank you guys for trying to help me but I just beat him. I redistributed my skill points to get double shurikens and mirage and that's what helped meh.