If he is taking his ease, give him no rest. If his forces are united, separate them.
Everyone needs rest. Even being alert and ready to defend is draining, let alone the exhaustion that battle engenders. One way to weaken your enemy is hence to constantly harry and pick at them.
Random small attacks creates uncertainty and keeps them tense. In this way, a small and nimble army can defeat a much larger army. Guerilla war is one such method.
Concentrated forces are very difficult to defeat as front-line troops can quickly be replaced with reinforcements. If you can break them into parts or spread them out along a long front, removing the ability to reinforce, then a single, small victory can provide you with a powerful gain. In this way, divide-and-conquer is a common and powerful strategy.