If you're playing a ranger, did you at least do the Ifan quest line until Zaleskar (assuming you got him)? You just have to talk to Borris in front of the gate to the magister place and then talk to Zaleskar for an op weapon. His crossbow is so ridiculous in Act 1 (Level 5, 8-11 piercing + 20-21 Physical. Pure physical damage!). Add in Blood arrow element and you will be dealing 80-90 a shot without considering the power of Lone wolf.
I don't even have assassinate yet but man, just a shot from that along with the AP on your first turn, you can pretty much down any boss armor and knock them down before they even act.