I don't think there's any way that Luffy can beat Big Mom.
He couldn't take her commander without Nami debuffing his shields. Meanwhile, we still have 2 other commanders to defend her. Luffy has only his weakest teammates with him other than Sanji...but even though Luffy is about to get a meal from Sanji, when has he ever been this weak before facing off with the main villain, whose been calmly going about her day like normal and hasn't had to expend energy at all (unlike Doflamingo who had to spend all day dealing with Law before taking on Luffy).
Of course, OP has bullshitted me before. It's perfectly possible that Sanji's meal is just gonna instaheal everything wrong with Luffy instead of only partially helping like it should. If it goes that way, it might be that Luffy has a chance to get some good shots in. But I hope it's only enough to give him a fighting chance. I don't want to miss out on a Luffy vs Big mom fight.