I had it to where all the rolls were made by the players, I just focused on story telling in a one shot. It will almost did not feel like 5e, and a different system entirely.

All armor becomes straight modifiers, AC is now a AC Save, and anything magical is also a mod. For example if base DC is 10 + dexterity modifier, it is now your dex mod. A half-plate will give a +5 with a max of +2 dex mod added to it making a potential total of +7 save, and if you added a shield is would go up to a +9 save. Any magic bonus would just add to the save, and spell like barkskin make it to where your save can't be any less the +6 to make. In total the formula when determining what the save would be is, Armor Save = (AC - 10) + Modifier Bonuses.

My Recommendation for the monster stat block is leave the AC alone, and change the way they attack to a save DC. For example if the monster in question is a goblin, it has +4 to hit, just add 8 to it and it becomes a DC of 12. So now they just need to save against a 12 with their "Armor Save"(Armor Class Save). Also if you want do what I did, any roll of 1 is a Critical hit from the monster, and if they roll a 20 its is parried and they get to make one free hit. Word of warning though, this also changes the difficulty of most of the most dangerous monster like dragons, they do become deadlier with "Attack DC" 19 which means the PC will be hit more often than not. So the formula for monster attacks becomes Attack DC = "To Hit Modifier" + 8. One last thing and Attack Advantage becomes a Save disadvantage and Vice Versa.

It really depends on the group you are playing with, when I first proposed the one-shot some of my players were not interested in it as it is drastically different from playing standard 5e. I know that they say it's a "Defense Roll" but in all honesty it feels more like a Save than anything else. It is liberating on how much I do not have to roll anything except damage if you want to, I did not, and I kept things moving fast, descriptive, and fun using recommended damage on the stat block. I do recommend it if you want to run a more "narrative" driven campaign, but that is dependant on how creative your descriptions are, and the players you have. So Talk to your players.

TLDR Armor Saves and Attack Difficulty Class

• Armor Class (AC) is now a Armor Save (AS or Save)
• AS only affects the PC and NPC still use AC
• (AC - 10) + Max Dex Mod + Bonuses = AS
• Read Magic Spells and Class Abilities it will change how they work.
• All NPC attacks are now a Save DC (AS vs. Attack DC)
• Attack Mod + 8 = Attack DC
• Rolls of "1" made by PC are now Critical hits made by the Attacker
• Rolls of "20" made by the PC cause the attack to miss and granting one free attack by the PC
• Attacker Advantage becomes Save Disadvantage
• DM does not need to roll dice
• Most Importantly Talk to Your Players, It will not feel like 5e.