If we gonna compare on "battle scenes"
Onigashima clears neg diff....
One of the major reason entire fandom were hype about God Valley is they were looking for the battle scenes which supposedly involved Roger + Garp + Xebec + all other legendary pirates and more.
And it was mid asf.
Being a flashback arc doesn't excuse anything as this is the final showdown of God Valley.
Roger and Garp aren't some random guy, and it's the hardest battle they fought in their career and going all out fight, there aren't any more detail, Oda shows the flesh and bone about GV and this what we get lol
By writing, lore and amount of info we get and Xebec character carrying the flashback, GV is better than Onigashima on that field.