Arachna is mean, but that bitch has no HP... Hit her really fucking hard, and if that spider comes out don't run from it, just fight it.... It IS faster than you...
You just gotta stun and gank like any other class, I was fucking her up pretty bad last night... You can't let her get too overleveled though, because once she gets hard to kill due to levels/items, she's unstoppable...