Hi I'm a Shen one trick with mastery 7 and I'm making this guide so people can further understand Shen's mechanics and playstyle.

Pros
+Map range shield and teleport
+Excellent split pushing
+Is energy based

Cons
-Ult doesn't give you extra 1v1 potential
-taunt and ultimate have long cooldowns
-Energy doesn’t scale

One of the most strongest part of Shen's kit is his passive, Ki Barrier. Which makes it so all his abilities grant him a shield on cast. Shen's passive is quite a short cooldown, but can also be lowered if Shen impairs an enemy. So if you utilize this fact, then his Ki Barrier can be used twice or more in fights. Combined with Spirit's Refuge, this makes it so most trades favor you.

Shen's strongest point is after he's level 3, combined with Titanic Hydra Shen can easily one shot squishies.
During laning phase, look for every opportunity to try to pass the blade through your enemies via your Twilight Assault. Eventually, you'll chip away at their health and can kill them easier. If you can, try combining the Twilight Assault damage with your Grasp of the Undying damage.

Post level 6 you'll want to look for Teleport with your ult. Ulting someone with Stand United while they're in a sticky situation can really turn the fight around. When ulting someone, keep in mind their escapes and when to Shadow Dash or not. If you know they have a dash or flash up, then Shadow Dash after they try to escape. If they do get away you can even try to do a Flash + Shadow Dash.

Grasp of the Undying is a excellent rune for Shen. Provides extra health and extra damage especially early game. Grasp of the Undying synergies well with Twilight Assault and Ki Barrier, So you win almost every trade.

Shield Bash is great on Shen when combined with your passive Ki Barrier. It will give you extra damage when you use an ability which would most commonly be used with Twilight Assault.Which is excellent for trading and poking.

Second Wind is a great rune for almost every top lane champion with the extra sustain you'll get for after your small skirmishes. Synergizes well with Revitalize so you will get more health out of Second Wind and will be able to stay longer to farm.

Revitalize is an overall good rune for Shen. It provides an extra shield for himself and his teammates with Ki Barrier and Stand United. This can help you win fights early game because of the bigger shield you will have.

Cheap Shot is a good rune for Shen when used with Shadow Dash's taunt or the slow from Twilight Assault to get in a little extra damage from a all in or just a small poke. With the true damage it gives may not seem like a lot but any amount of damage matters and can be the difference between losing trades and wining trades.

Ultimate Hunter is a must on Shen because it will reduce the cooldown on his Stand United since he has one of the longer ult cooldowns in the game so you'll want it up as fast as possible.

You'll mainly want to take Ignite when you either play support or you want to be aggressive in a solo lane and don't want to roam that much and focus more on split pushing. Ignite will give you extra 1v1 potential but you'll lose out on some objectives if you don't take Teleport.

Teleport is great on Shen when you use Stand United you can teleport back to lane if needed. You can also use teleport in place of your ult when its on cooldown you normally always want at least one up at all times if possible to help your teammates.

With Flash it is a must on Shen. It pairs very well with you Shadow Dash and doubles the distance of it in a way. I'll talk about in combos.

With Smite you always take when you jungle.

Ki Barrier is Shen's passive that gives you a shield for 2.5 and cooldown of 7.5 seconds but is reduced to 4 seconds if you CC's an enemy and is crucial to play around this early game because it will help you avoid taking a lot of damage.

Twilight Assault is Shen's q that gives you 3 empowered auto attacks and brings you spirit blade to your location, damage will increase if it passes through an enemy, it will also slow them for 2 seconds

Spirit's Refuge is your w, it creates a circle on the ground that can block only auto attacks that lasts for 2 seconds. To move it you'll need to you use Twilight Assault.

Shadow Dash is Shens e. You dash in the direction of your cursor and taunting enemies passed through for 1.5 seconds this ability is what you will use to either engage fights or disengage fights or even to keep an enemy from running away.

Stand United is Shens ultimate you teleport to an ally shielding them for 5 seconds. Depending on the targets health will determine the size of the shield. It’s great for securing objective, changing team fights, and saving allies.

First Combo is the Flash taunt
Flash + Shadow Dash
You should only use this if you want to close distance on an enemy and you no you can kill them and is risky because you would have no escape if you need it. you could also use it to run if needed.

Second combo is with Titanic Hydra
Shadow Dash + Twilight Assault + AA Titanic Hydra + AA + Spirit's Refuge
this will be you basic combo and what makes Shen Extremely strong in fights. You can use Spirit's Refuge at any point in the combo to take the least amount of damage. The key is to have your spirit blade pass through the enemy it will give you extra damage and will slow them if they run away from you.

The third will be with your ult plus the previous combo
Stand United + Shadow Dash + Twilight Assault + AA + Titanic Hydra + AA + Spirit's Refuge
This combo you'll mostly use to save your allies or to initiate a fight.

As early in the game you want to have short trades with your opponent if they are strong early game and you want to proc Grasp of the Undying as much as possible as it will do extra damage to your opponent. Other than that you mainly want to focus on farming, to note Shen’s early game wave clear is not that great since he has no aoe but that will change when you buy a Titanic Hydra or a Sunfire Aegis.

Shen’s mid game is much easier than his early game depending on how you farmed your should be much more of a tank now but don’t get to carried away and do 1v3 as mid game you will mainly only be able to pull off a 1v2 depending on how you play it but you want to look for ult as in securing objectives, and changing teamfight.

The late game is where you really excel because you’ll be a full tank that can do a lot of damage and can easily save allies and possibly 1v4 depending on if your fed or not but if you are a little behind just wait to fight with your team and you want to be initiating the team fights and try to taunt the enemies back line and take out the adc as quickly as possible since you should be able to 3 shot the ad carry with your Twilight Assault you want to focus everyone but the enemies tank.