The best advantage a spy has over a merc is height. If you can stay above him, you can set up an ambush. You should always avoid combat with mercs until you're sure you can take him out. When you're moving about the map, go slow. The merc's detectors will easily pick you up when running, and they'll intercept you and cut you down. Only go fast when you're taking out a merc or trying to get away after you've already been detected. If you're discovered, drop a flashbang and run. For mercs, always pay attention to your detectors, they're your best friend. Your other best friend is the proximity mine. Since Pandora Tomorrow, the proximity mine has been the most important piece of a merc's defense. You can use these to cover common paths spies take or to cover objectives when you aren't there. Another tip is to not run around the whole map. Patrol the general area near the objective, and try to keep a good vantage point.