I will typically use stealth before engage becuase you can flank an enemy and won't show up on radar.  At which point, when you stealth wears off you are already behind them to rocket/blast them in their rear.  From that point I will either uses the mech's high speed to run circles around the oppoing mech until my ability and weapons reload or will run away and repeat the process.

I also try to target mechs that have already taken damage as it does need a little help to finish someone off especially at close range.

Up grade and run missile rack 6. Each trigger pull your dump your pay load.. and find reload time is relative to ability recharge.. avoid shielded mechs, and aim for flanks and exposed sides. 

Opt to target damage mechs and mech currently under attack for max efficiency 

I agree that MR6 or even RPG6 is the best choice for a Shadow.  You won't be getting a good amount of kills but it is a fun loadout to play.

Recently, I have been running Javelin 6's on my Shadow and using steath as a way to get around the map and as a defensive ability.  With rank 7 Javs you can get a decent amount of kills as long as you can stay alive.

I play it with missile rack 4 at the moment. I either guard 2 points and fire from behind cover or i flank at cap points and fire from behind cover. It's becoming my fav mech.