I am a very confident and aggressive shen player and have always come off well late game, however your moves especially your taunt is paramount to do well as if you miss this you could well like lose your fight.
Shen is the kind of player where the enemies try to ignore you but you draw them in with you taunt, and you are either likely to be the last one in a teamfight left or you are going to get targeted early because of your taunt.

In my runes it says the per level MR runes however i actually use the flat MR but the option is for some reason unavailable on here also i use a Magic pen quint instead of Ability power it just was not there to choose from for me.

I use the same masteries in all builts as the tankyness and damage is always expected of shen even in support role. However you can swap a few round for instance for the energy mastery in utility if you need it, however i find shen energy return is very good especially on his taunt so MAKE SURE YOU HIT!!

Summoner spells can be switched out as you see fit, Exhaust is very useful on shen and can be swap for flash when top and you have Shadow Dash to escape also summoner spells such as ignite, ghost and heal are all useful spells for shen.
Ignite is useful as it can stop your enemy for restore HP and can be used to secure a kill.
Ghost is good for those time where they are just too far away to taunt and you want them dead also if you wanna chase that teemo you can have your movement speed buff too.
Heal is useful as support for your carry, in teamfight to boost your team, in ganks to help your teammate and in lane if you need to stay longer.

Pros
Multi-opponent taunt
You are practically unkillable
Global savior ulti
Your team will love you for your saves

Cons
If you miss you taunt, you will lose

Summary is that shen is a Boss and if built well can do alot of damage aswell as being almost unkillable. I rate shen as one of the best league champs to date and lets hope they dont nerf him again.