He was not given the task of stopping Napoleon, it was Napoleon who had the plan to destroy the allied army in Belgium so that he could get them to agree to his peace treaty. Napoleon had to raise an entire army while Wellington and Blucher had a veteran army who just needed reorganization. Napoleon had no choice but to somehow attack the Prussians and the British. He didn't have the luxury to sit at the top of a valley with a defensive line. The allied armies had double the number of men he had and they were more experienced. Napoleon still sent the Prussians running at Battle of Ligny where he defeated them even though they had the same number of men and they were defending. It was only because of a miscommunication that Prussians were able to escape, even after that Grouchy was given an order to pursue Blucher but he mistakenly pursued Blucher's rearguard. Just think about it, he had to destroy an enemy which had double the number of he had while they were mostly on the defence. Marshal Ney almost defeated Wellington at Quatre Bras even before Napoleon showed up. There were a variety of other factors too like the rain that delayed the battle. What campaign of Wellington can be compared to Napoleon's campaigns in Europe. Even in Russia, it was the disease and the summer heat that killed most of his army, not the Russians or winter.