I like this match with Reptile, it's not as bad as it seems on paper. Here's why:
REPTILE
With instant acid spit you can time it so you don't get hit by Shang's fireballs. Reptile's slide will sneak under those fireballs most of the time as well. A good strategy is to run away, go invisible, and then see what Shang does and react to it. It's relatively easy to anti-air roundhouse Shang with all the ninjas as well, which negates one of Shang's big weapons. Use forceball sparingly, and from afar. But DO use them, because they get Shang jumpy. If he morphs into top tier, adjust your strategy likewise. Try to anticipate when he will morph back and try to make him pay.
SHANG
The fact he can basically be Mileena Jax or Kang the entire match, definitely gives him the edge. But with Shang's sick CQC that destroys ninjas, you don't even need to unless you get in a tight spot. Fireballs are a good weapon if Reptile wants to trade projectiles, but beware the slide. Again, Shang almost has TOO many options in this match, and that can get you into trouble sometimes. Try to keep Reptile visible with an array of fireballs. If he does go invisible, a good safe morph is Lao and then teleport to find Reptile. Most Reptiles won't be able to uppercut you out of that because they'll still be processing that you morphed into Lao. Jab and sweep if you can get in. Shang walks so slow that it might be hard to track Reptile down, but be persistent.
I'd say it's 7-3 for Shang. An elite Reptile will give Shang some problems!