I would push STR as high as you can and look for Adept III or better of your main backstabs and buffs/offensive stance as priorities. I did so and became very effective, and I am just 45 on my Assassin so not too far off of where you are. I think my STR comes in around 300 or so, but I have some pretty decent gear. Then I would examine the various casting order threads going in here to determine how to maximize what you have to best effect.