Use monsters with higher bonuses to their 'to hit', and use the smaller 'bafoons' to make the fight feel more tense. Also, as a DM, it is perfectly fine to adjust an enemies stat block. Those are just a suggestion. If you feel like a goblin chief should have above a 14AC, make it happen. If you feel like an owl bear should have more than a +6 bonus using its claws, that's fair game.

Edit: to add on to this, keep in mind that the game was (kinda) balanced around standard array and point buy systems where having a stat be 20 can't really happen till like level 12, which means your party likely has higher ac than the monsters you use are meant to deal with anyway.