Ok here's the problem: i've got a cleric. AC 18, and, he is pretty untouchable. no monsters can harm him. am I doing something wrong?

edit: the cleric's level is 3

Heavy armor and a shield? 

No, nothing is wrong - you've just been kind of lucky because you haven't gone up against many opponents that make you roll saving throws instead of attack your AC, and the dice rolled for attacks against you haven't landed on higher numbers.

Exactly. 

AC 18 shouldn't be that untouchable.

For example, an Orc, a CR1/2 monster, has +5 to hit. That means to hit an AC18 character, the orc would need to roll a 13 on a d20 - this should happen about 40% of the time. That's a low enough level monster that your party should be fighting many of them at once, if your characters are 3rd level. At that level you can encountering monsters that attack multiple times per round - for example, the Redcap, CR3 (so a valid challenge for a party of 3rd-level characters) attacks 3 times at +6 to hit, so it should usually land some hits on the cleric if it gets a round to attack. Are you doing something simple like forgetting to add the to-hit bonus for the monsters, or forgetting about monster multiattacks? 

That said, a smart party can improve the odds a lot. For example, if you've got a spellcaster in your party that's playing a supporting role, it's quite possible that lots of monsters spend a lot of time attacking with disadvantage, and yeah, monsters attacking with disadvantage against a CR18 aren't gonna hit very much. Or even just wasting their turns escaping from webs or mind control or whatever debuffs they've been cursed with. That's probably normal then, a good defensive spellcaster doing their job well in combination with a high AC character is a pretty potent combination, as it should be. 

Or smart parties can find ways to set up ambushes and stuff to make sure they often get the first strike and the monsters don't have much of a chance to even take swings - if usually the monsters get 1-2 swings on the cleric per fight, then it's not surprising that it feels like the cleric only gets hit like every other fight. (If this is the situation, then the remedy is to give the party some more intelligent monsters to fight! While a zombie army might shamble blindly towards the PCs, groups of bandits or fiends can also do things like set up ambushes, use cover, and in general use tactics smarter than "run forward and swing". )

As the others above said, it's nothing you're doing I wouldn't think.  Could be simply the dice your DM is rolling falling in your favor more than normal.  You'll go through spells like that sometimes.  I've had low AC characters that seem untouchable to some extent because of low dice rolls going in my favor when critters/NPCs attack.  But then, I've also had high AC characters that you would've sworn were practically naked they got hit so much sometimes because the DM would be on a roll and hitting high numbers left and right when rolling attacks against me.  As long as you're following the rules you're fine.

Quote from morninglord2 >>

As the others above said, it's nothing you're doing I wouldn't think.  Could be simply the dice your DM is rolling falling in your favor more than normal.  You'll go through spells like that sometimes.  I've had low AC characters that seem untouchable to some extent because of low dice rolls going in my favor when critters/NPCs attack.  But then, I've also had high AC characters that you would've sworn were practically naked they got hit so much sometimes because the DM would be on a roll and hitting high numbers left and right when rolling attacks against me.  As long as you're following the rules you're fine.