Takedowns on enemies grant permanent Attack Speed.
Offense
+9% Attack Speed
Flex
+6 Armor or 8 MR (match-up)
Defense
+15-90 Health
WhyGrasp of the Undying
Shen naturally takes great advantage of Grasp's empowered auto-attacks thanks to the majority of his power being in his [Q].
WhyShield Bash
Shield Bash on Shen is easily proccable with [Q] and synergizes with his wanting to auto attack for the majority of his offensive damage.
WhyConditioning
Conditioning is an all-around solid defensive rune, giving up early weak resists for stronger resists later. These resists also scale well with Shen's natural health stacking.
WhyOvergrowth
Shen loves to stack health as it improves his passive shield and tankiness. Overgrowth scales incredibly well with high-health items and champions.
WhyTriumph
Triumph's massive mid-fight heal allows you to sustain incoming damage in teamfights so you can continue front-lining.
WhyLegend: Alacrity
The extra Attack Speed from Alactrity is fantastic for trading in lane with [Q] empowered auto attacks.