AC 20 isn't unreasonable and isn't that hard to hit.
At level 2-3 most characters should have at +5 to hit (+2 from proficiency, +3 from main ability bonus) on their 'bread-and-butter' attacks. That's a 25% chance to hit without throwing in some tactical play and use of special abilities (which are worth spending on a fight like this).
- Faerie Fire gives all attackers advantage if the target is affected. That improves the probability of landing a hit to 43.75% per attack roll for everyone in the party.
- A monk gets up to 3 attacks per turn with Flurry of Blows (That's 57.8% chance to land at least 1 blow, and 82.2% chance if those attacks have advantage.)
- Bless gives an extra d4 on attack rolls.
That alone improves the probability of a hit landing from 25% to 37.5%.
- A druid in spider form can Web an enemy to apply the Restrained effect, again advantaging all attacks against the target.
The list can go on, but these examples should suffice.
If the fight is challenging, it's time to burn special abilities. And then statistically a party of 4 should be able to average 1.5 to 2 hits per turn between them.
Aside, it's worth bearing in mind that sometimes an option is to avoid the fight altogether. Your party could try talk their way out of it, or even run in a pinch.