The Scots, under Robert the Bruce, were besieging the castle of Stirling in 1314. The English sent a gigantic army under the direct command of King Edward II to relieve the besieged castle, but King Robert arrayed his forces on top of a hill that was behind a marsh. The hill overlooked the town and castle in such a way that the English would have to attack the Scots where they had arrayed themselves if they were to raise the siege.
The Scottish army was mainly composed of heavy infantry armed with pike, although there were a few light infantry bowmen as well. They stuck a few of their heavy cavalry, dismounted, in with the infantry while 500 remained mounted in reserve with the king on a particularly high hill.
The English had to bring their enormous army across the marsh to attack the Scottish army, but they managed to do it by the morning of the battle. They hadn't gotten the heavy cavalry set up for battle, though, and the infantry wasn't even on the field when Robert's troops attacked them.
He started by moving the pikemen off the hill and across the marshy field to attack the relatively unprepared English. The English saw them coming and tried to countercharge, but they did so to little effect. The Scottish infantry held fast and continued to hold their own in the ensuing battle. There was a brief period when the outcome was in doubt, however, when the English managed to get a detachment of light infantry armed with the formidable Welsh longbow on the Scots' flank. Robert was well prepared for just this threat, however, and sent his 500 mounted cavalry that he had held in reserve in to disperse the bowmen.
Heavy cavalry being superior on the attack against light infantry, the bowmen were quickly eliminated as a threat in the battle when they all took off and hid. The rest of the Scottish heavy infantry then joined the battle and the Scots effectively cornered the English against the marsh behind them.
King Edward II, who wasn't really any good at this kind of thing anyway, picked this time to exit stage right. The rest of the English army tried to do the same, but not too many of them made it across the marsh. The English suffered heavy losses and the Scots effectively won the battle.
Robert effectively outgeneraled the outmatched Edward. In dismounting the majority of his cavalry, he defied convention, as he also did by attacking with pikes. The men held in reserve, while known and practiced since ancient times, had fallen out of favor in Robert's day. By using these strategic moves, Robert was able to overcome his opponent, whose strategy mainly consisted of riding up on a horse and hitting the enemy with something sharp.
This account taken from Archer Jones' account in The Art of War in the Western World