My initial reaction while reading the chapter was Rebecca awakened Conqueror´s Haki or something like that, but it was probably just her strength. It makes no sense to have a whole chapter about Toy Soldier who repeatedly states his oponents being "slow" (we only saw such a thing from Kizaru and timeskip Luffy recently) talking about Rebecca and on the forelast page stating he taught her nothing beside "strength", I mean…
But at this point, seeing what kind of surprises we have experienced so far in the arc, it could be absolutely anyone that ever appeared in the story so far. Going by Occam´s razor logic though, the simple fact that the commentator stated everyone was down but right afterwards someone was beginning to stand up, means that the cause of the blows may very well be one of the people we saw lying on the ground beforehand. In fact, it is heavily implied and going by the chapter build-up, the most likely candidate is Rebecca.
Anyway, I was laughing hard at Usopp and his bravery as soon as he saw how mighty the little guys are and that he probably didn´t have to lift a finger to pull off the hero here, reminded me of Buggy
Have fun everyone and let´s stay sane until the next chapter.