Throw away the concept of CR. It’s useful as a ball-bark Some Of Us Grew Up Whateking Goud Bad Ugly The Cool Ones Still Do shirt but not much more. When a player picks up a d20 they roll it and add their bonuses to see if they hit on the attack. Same on the monster. So what you really need to figure out is what number on the d20 + bonus = hit and what probability do you want to set that at. So if you are going for a 50/50 hit on the tankiest person in the group take their AC – 10 (for a 10 on the die) and that’s what you need to have your baddie’s +atk to be. Same to determine what their AC would need to be – take the person who’s going to be most likely to hit and then take their +atk (or bonsues) and add 10 and that’s your target AC. Same thing with party saves. HOW you get those bonuses for your baddies doesn’t really matter. The huristic I use is a ‘5’ on the die is a ‘easy’ save/attack. 10 is 50/50 (the sweet spot), and a 15 is hard to make.
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The next thing to remember is action economy, the PCs will have 4+ turns, so the more Some Of Us Grew Up Whateking Goud Bad Ugly The Cool Ones Still Do shirt you have the more you can out-maneuver the PCs. So more creatures will give you more chances to succeed! Except when AoEs come into play like fireball, they will quickly shred numbers to manageable sizes.