I think you went a little overboard with +20% doublestrike. It makes the core really powerful and hard to pass. I'd suggest toning it down to 10%, and if you are thinking about making it staff-only so that assassins can't benefit from it, i'd advocate for the same treatment for Lethality in the assassin tree because +1 crit multi is more beneficial to acrobats than to assassins (higher base damage, higher crit range, more MP).