How to beat Shen as Renekton:
Shen is probably running Grasp with full tank masteries. Run Fervor. Always try hitting him with empowered Q's to beat his sustain with Grasp. Harass with Q Level 1 and last hit to build fury. Mind the level 2, Shen might try and E-Q to draw minion aggro. If he misses the E, punish. Don't hit Shen when his shield is up, you'll waste fury that way. Once level 3 hits, it's pretty similar to the Fiora match up. Baiting the W is everything, but his W doesn't stop your Q or E. When he blows the W, unload with E Q AA E AA W AA, but always mind the spirit sword because his shield and damage are annoying if he gets the pull-through. By this time he should be chunked using potions. Zone him away from the wave to deny CS and EXP as much as possible. Pushing him into the turret is an option, but it's risky as Shen's Q lets him farm easier than before (his waveclear is still mediocre before Sunfire) and he can draw turret aggro and set up a gank with E. If he tries to ult or TP across the map, E W or Flash W him to interrupt the channel. Mind doing this next to a turret, he'll probably re-engage with E to draw aggro again.
It's all about playing around Shen's areas of strength (The E, the shield, the W, the ult). Good positioning is key when going against Shen. A good Shen goes to the rest of the map, a good Renekton brings the map to him :^)
Shen is really strong tho tbh, no idea why very little competitive play tbh. That W is broke