I recently started using encounter generators and tables. I've noticed that there are encounters like "8 Cave Spiders" or similar. While the number of enemies will result in an encounter that "should" be challenging, each creature's turn is limited to a 1v1 encounter with a player. Because of this, the player's statistics tend to make combat futile for creatures under a certain level (can only hit and save on natural 20s, can only affect players with spells or dodge players attacks on their natural 1s).
Now, is there a way to take creatures that fall into that category of 5%ers (1 in 20 chance to do anything) and make them do something useful more than 5% of the time under RAW?
I'm not asking how to modify the encounters to produce creatures that aren't 5%ers. I'm asking how to take those encounters as written and make them as challenging as they're supposed to be.