The problem with looking at them as separate battles is that it makes you draw the wrong conclusions about each. Historical analysis of each individual action leads one to conclude that Ney was incompetent, Napoleon was sick or past his prime or not paying attention or delusional, etc. and made horrendous mistakes. Grouchy was lazy and stupid. Wellington was a genius, and Blucher was a sideshow. If you take the old fashioned psychological/behavioral nonsense out of the analysis you can make a much more satisfactory conclusion about it.

Napoleon knew Blucher was coming. He was expecting Grouchy to head them off. Instead, Blucher arrived with 30,000 men and made a flanking attack on Napoleon's right and the battle was essentially over. It took too long for the grand battery, in muddy ground, to get set up to bombard Wellington's left center. The strategy was sound and classic Napoleonic tactics. The implementation and execution sucked, starting at Quatre Bras, because the army was formed too quickly and didn't have the cohesion earlier French armies had. The marshals did not know their troops at all, and their troops were suspicious of their generals. If you were fighting under Ney, who a couple of weeks ago said he would bring Napoleon to Paris in an iron cage, you wouldn't trust him either. I think that the fact that the army had no time to train and get used to their new generals before the battle led to poor execution of orders. Could Berthier have made a difference? Maybe, but with such troops, probably not.

"Nevertheless, when all was said and done, he remained a giant surrounded by pygmies; his reputation survived his fall, for his basic greatness was inviolable." (David Chandler - "Waterloo - the hundred days" p 41)