We got to talking a bit about CR – that’s Challenge Rating – a bit in Discord the other day; Demagor, Andi and I. CR is not really a useful measure of how dangerous a given opponent might be, though. The example creature in this case is an imp; a minor devil. On the face of them, Imps are tiny nuisances, barely a threat to a party. They don’t do very much damage, and unless you get a swarm of them, they’re not much of a threat.
Or.. are they?

In direct combat, this is true. But throwing an imp at a group in a direct combat does not play to the imp’s strengths. I talked about that a year or so back in this post; and I didn’t really talk about it in much detail at the time, so I’m going to expand on that today, using that imp as an example.
An imp is not a direct combat monster. Their abilities include a naturally high stealth, small size and invisibility at will – some versions can even shapeshift – making them best suited for covert operations. This is a natural scout or spy, capable of ruining a party’s best-laid plans just by hanging out in earshot. Being a rat under the table, or an invisible presence in the rafters, an imp can overhear plans, and get to know a party better than they know themselves.
Then they can use that knowledge. Tip off a larger enemy. Foil an ambush. Cause confusion in the ranks, or worse.
If forced to fight, their poison can knock a mage – usually a party’s heavy hitter – clean out, using invisibility and a their poison stinger. Combine Invisibility and Suggestion and you have dissent in the ranks as the fighter or rogue keeps the healer from helping their fallen companion.
And an imp is only CR 2. This is a major nemesis for a low-level party without access to Glitterdust.
I have seen GMs run fights stupidly. High CR monsters played badly get torn to pieces because they did not use their natural advantages and most high level parties absolutely will use theirs, if they are smart. If your monster can fly? Get him airborne! Force the party to bring him down. Strafe. Grab and drop. And give the party an opportunity to flee, if necessary; not every fight has to be won for them to learn something from it, and an opponent may not be interested in killing them right away. That imp may prefer to play with them; either from just being an evil little monster or from being on orders from a higher power, or he may be collecting information to sell to the highest bidder.
This right here is just one illustration of how a comparatively low CR monster can be a major hassle for a party. I’ve seen low-level groups get completely trashed by, of all the things, a mere animal. I’ve seen mid-level groups get pounded by simple monsters because they had the better position and used it. And I’ve seen high-level groups get turned to paste because they didn’t take strategy into account and had it used against them.
As the GM, remember, your NPCs are /your/ characters. You can – and should – play them to their strengths. Make the party work a little to defeat them