As others have said - as the DM, you are the ones placing monsters and NPCs in the world. The PCs should not normally be encountering single, ordinary monsters as a genuine encounter. If they do, since you placed that enemy there for whatever reason, award xp as normal but don't count it as a milestone. Simple as that.
But that said - what are the circumstances under which it could be reasonable for PCs to encounter a solitary foe?
1) The PCs specifically go out of their way to do so - for example, rather than confront the guard captain while he is surrounded by his soldiers, they wait for him to head home and then ambush him in an alley. He's an ordinary fellow, and goes down easily. There is no reason not to give out normal xp for him - the PCs specifically set up the conditions that led to him being easily defeated, so deserve an easy reward.
2) The PCs are in a situation where single enemies are roaming about. Perhaps they are escaping from a fortress which has guards patrolling the corridors. Rather than have them fight a bunch of easy fights against 1 ordinary guard of their level, it is up to the DM to be a bit inventive here. I can see two ways to handle this.
First, make all of those guards minions. The PCs are now engaged in an ongoing event of avoiding enemies - and if they do encounter a foe, they need to take it out quickly so the guard doesn't set off the alarm. Fortunately, minions go down in one hit - and they also are worth a very small amount of xp, so the reward is suitable for the ease of the task. Though easy to knock out each of the guards they get caught by, the risk - of triggering an alarm and much more dangerous battles - makes it a battle worth giving out xp for.
Alternatively, map out an entire section of the fortress as one encounter. Say it consists of 10 guards - 3 of them are one solitary patrols through the corridors, 4 are playing cards in a nearby room, 2 more are resting in their barracks, and the last one is filling out some paperwork. As soon as one guard is encountered, combat starts as he calls out for help - and while he might be taken out easily enough, the rest of the combat will arrive in the next few rounds and, as a whole, it provides an entirely legitimate encounter.
They key is, in the end, that you as the GM are the one setting these encounters in place. Any battle that should be a genuine fight, design as a genuine fight. If you need to have solitary enemies walking around, make them low level minions, or make them part of a larger encounter. If an enemy isn't going to be combat relevant, it doesn't even need to be a speed-bump. And if the PCs manage to, on their own, arrange a way to confront enemies one by one, then good for them - reward their accomplishment by giving them normal xp despite the ease of the fights.
But that said - what are the circumstances under which it could be reasonable for PCs to encounter a solitary foe?
1) The PCs specifically go out of their way to do so - for example, rather than confront the guard captain while he is surrounded by his soldiers, they wait for him to head home and then ambush him in an alley. He's an ordinary fellow, and goes down easily. There is no reason not to give out normal xp for him - the PCs specifically set up the conditions that led to him being easily defeated, so deserve an easy reward.
2) The PCs are in a situation where single enemies are roaming about. Perhaps they are escaping from a fortress which has guards patrolling the corridors. Rather than have them fight a bunch of easy fights against 1 ordinary guard of their level, it is up to the DM to be a bit inventive here. I can see two ways to handle this.
First, make all of those guards minions. The PCs are now engaged in an ongoing event of avoiding enemies - and if they do encounter a foe, they need to take it out quickly so the guard doesn't set off the alarm. Fortunately, minions go down in one hit - and they also are worth a very small amount of xp, so the reward is suitable for the ease of the task. Though easy to knock out each of the guards they get caught by, the risk - of triggering an alarm and much more dangerous battles - makes it a battle worth giving out xp for.
Alternatively, map out an entire section of the fortress as one encounter. Say it consists of 10 guards - 3 of them are one solitary patrols through the corridors, 4 are playing cards in a nearby room, 2 more are resting in their barracks, and the last one is filling out some paperwork. As soon as one guard is encountered, combat starts as he calls out for help - and while he might be taken out easily enough, the rest of the combat will arrive in the next few rounds and, as a whole, it provides an entirely legitimate encounter.
They key is, in the end, that you as the GM are the one setting these encounters in place. Any battle that should be a genuine fight, design as a genuine fight. If you need to have solitary enemies walking around, make them low level minions, or make them part of a larger encounter. If an enemy isn't going to be combat relevant, it doesn't even need to be a speed-bump. And if the PCs manage to, on their own, arrange a way to confront enemies one by one, then good for them - reward their accomplishment by giving them normal xp despite the ease of the fights.
