BB, allow me to you some advice friend.
Try this for your s/r combo ->
(If you can) start by practicing being the first one hitting the boss. With snipe. (this is pretty huge)
You should really start buffing way earlier than what you do. Blood arrow should be the first thing you put on as far as "poppin' CD's".
Try, Snipe -> sapping -> engage autoshot + shot -> combo shot -> (Option A1 [you crit with shot]) vampire arrow -> snake poison bite -> weak spot -> mana drain shot -> shot/WA -> continue with any of the skills between first shot and last shot(/WA) immediately after CD is finished.
Option B2, use shot until exploiting shot procs
Option C6 (the boss you are fighting spawns mobs)
combo shot -> (Option A1 [you crit with shot]) vampire arrow -> snake poison bite ->
Prioritize piercing arrow and reflected shot.
S/r has a lot more group target potential than most scouts. With the upgraded PA and Reflected you can increase your deeps by doin this.
Just some tips >.>