Player: “Ok, I’m going to move 30 here and use my sneak attack on the last- oh wait, I can’t, nevermind.”
Me: “Wait, why?”
Player: “He’s out of range by 5ft, I have disadvantage so no sneak attack.”
Me: “Bro just roll sneak attack, he’s the last goblin. We have cooler shit to get to this session.”
Player: “So, advantage?”
Me: “Yes advantage. Please don’t make me play one more round as this 7HP cannon fodder.”
I personally will roll “bravery saves” for npcs that are in over their heads, its just a d20, 1-10 flee 11-20 fight, basically just a fight or flight instinct, or I roll a WIS save against the closest players passive intimidate
I roll it anytime an enemy: has its hp reaches a point where they will likely die to one more hit, has no more allies in line of sight, sees an ally die and now has less allies than active threats, is actively intimidated
fight means charging into combat to deal as much damage as possible before death
flight means running away, but it doesn’t necessarily mean giving up on whatever goal they had for ex. If the enemy was attacking in order to steal an item, a fleeing enemy may grab the item as it runs away