I was all ready to post but have one more question to ask first.
The plan is to launch attacks and then cast sleep. Simple enough. But . . .
Constantine is planning on launching a javelin. The javelin has a normal range of 30'. At greater ranges it is at disadvantage. Currently you are 70-80' away from the bugbears. The javelin can be thrown but will have disadvantage. The bows have no issues. Is the plan to launch from this point or to sneak closer first?
FYI - it is currently raining modrately hard and the terrain is wooded - with patches of rough. I will try to get a picture up shortly - but last time I tried it didn't work. The terrain isn't likely to change your plans much so I'm not overly worried about the image if I can't get it taken care of.
The current distance would also mean that Silver would take a couple of rounds to get to the otter from where you currently are located.
You are, of course, welcome to try and move up more but I would need to know how close you are trying to get. And, of course, there is always the chance of being seen.