I have two types of enemies that I fall back on if I don't have something interesting or appropriate prepared:
A. Powerful but dumb
B. Weak but cunning
Between those two types, you can create nearly every type of OSR creature challenge you'll ever need. The key is that both types tell you about how the enemy thinks, which is the main thing the PCs must interact with. When you look at a big fancy statblock for some monster from a new school "Combat as Sport" game, you don't have any idea how it thinks. Well, pick one of these two.
Type A Enemy: Powerful But Dumb
I had a party of six different level 1 knaves all on a quest to go hunt down a troll. They were terrified, and the further they got into this quest, the more reasons they discovered to be terrified. The troll has a ton of HP and decent AC, does a lot of damage with a basic attack, but most importantly, is really fucking strong. A player tried chasing it down alone and got a tree thrown at him, shattering his arm. When the party tracked the troll down to its lair, they watched it being awoken by a damn fool NPC knave, whose spine was then compressed like an accordion.
But the players killed the troll with not a single tree thrown at them this time. Why's that? Because they talked to it, and they lied, and they made it angry, and they kept distracting it, and so on. They did everything they could to play on how dumb it was. My rule of thumb for a Type A enemy is this: any type of trickery the players attempt against it will succeed by default.
Type B Enemy: Weak But Cunning
The most frequent candidate I use for this type are NPC knaves, because I like to show the players a dark reflection of themselves. Other common choices are any kind of monstrous humanoid, such as frog folk or hobgoblins. The key is that each individual member is either roughly as powerful as a PC, or less.
In this example, I had three different level 1 knaves enter a dungeon that had been set up as the HQ of a band of brigands. Long story short, they had worked their way into the center of the dungeon and had either killed or scared off each NPC they'd come across, funneling all of them towards one corner of the dungeon where their leader tried to coordinate a counterattack. There ended up being a standoff in two dungeon chambers with a closed door in between them. The players were desperately holding the door shut on their side, as were the NPCs. Neither realized that the other was not trying to barge in. But that gave both sides the chance to prepare a surprise attack.
The players lost. They were simply not as clever as the NPCs. When the door swung open, they saw a brigand training a musket towards the ground, and a gunpowder horn rolled to their feet. The gun shot and hit the horn while the door was simultaneously slammed shut. One of the PCs died in the explosion.
My rule of thumb for a Type B enemy is this: they play like an experienced, skilled player would in their position. Retreat, ambush, strength in numbers, leverage resources, and NEVER FIND THEMSELVES IN A FAIR FIGHT.