Hello there. The reaper has great burst capacities but in the end falls short when it comes to consistent damage upkeep. One of the problems I see in the spec is the cold shoulder trait. Keeping the chill up on a boss is probably possible, but forces you to delay or skip harder hitting attacks. I have two suggestions to make the reaper a bit smoother.
I personally would favor option 2 as it would enforce jumping in and out of shroud as a DPS mechanic but either would be fine with me. It would also make you decide to either pick up the damage boost or the crit boost, with both having good niches for different playstyles.