You are all underestimating Luffy. You guys are OK with any secondary character getting power up and becoming 3 or 4 times stronger without any reason but when it comes to Luffy, it's suddenly a problem.
Law faced Fujitora and Doffy and still managed to escape.
And the difference between Shiki and current Luffy is not that big.
If he goes all out from the start, using G4, FS and adv CoA, he can fight them for one chapter and do well until help arrives.