I'd recommend emphasizing Hide and Move silently in order to sneak up on them, and use a
Bag of Devouring instead of a
Bag of Holding. The Bag of holding states it has enough air for 10 minutes, you'd actually be more successful with a mundane, greasy burlap sack in that case.
The bag of Devouring has a 90% chance of ignoring the first presence of flesh upon first intrusion (so a 10% chance of activating), than a 60% chance of activating after that. So, drop in a piece of meat, or a small mouse, or whathaveyou in the round before you sneak up, than get the bag over the opponent's head. 60% Chance of activating per round, you're likely to only need to keep the bag on the head for a few rounds.
Creating a character with a strong grapple check, ability to use Rope, or quality manacles might also be helpful.
subduing your opponent beforehand with drow-poisoned sleep arrows, sleep spells, stunning effects, anything that might incapacitate your opponent for a few rounds and could possibly be done at a distance might also be helpful.
The idea has merit. If I was your DM, I'd let you pull it off a time or three before something just so happened to coincidentally go wrong with your bag.