While I am no expert on either of them here is my take :::
Both at the top of their game…
— Iron Fist before giving up his powers / at his traditional power level he has been at for years
VS
Shang Chi at the same time
… yes Shang Chi is often considered the best Martial Artist… though even that is questionable. Back in Heroes for Hire vol 1 #19 in 1999 … Iron Fist defeated The Cat. And while Shang Chi s often cos the best… Also The Cat is considered Shang Chi’s equal (yeah comics logic lol).
Iron Fist & Shang Chi often have fought to a stalemate lik in Iron Fist #7 2017 … they fought to a stalemate. But Shang Chi was being mind controlled… so like most of these fights usually heroes will hold back.
Just look up all of Iron Fists powers, they give him such an advantage. But the biggest advantage Danny has is was his popularity. Starting with Immortal Iron Fist vol 1 #1 2007. Danny has been through ALOT that really has put him on a next level to Shang Chi. Just to survive these insane inter dimensional fights .. battles.. war… And. To come out victorious, he had to push himself. Unfortunately, Shang Chi hasn’t had the spotlight lately long enough to help out. The Phoenix tournament really is just the biggest thing he has taken part in.
For me Winner :: Iron Fist
Buuut if it is right now Shang Chi Vs a depowered Iron Fist. Danny seems tired / done with being the super hero Iron Fist. He willingly gave up his powers. Shang Chi however is at the top of his game.
While I could see this being a stalemate… I likely is Shang Chi being victorious.