Give them cunning action (the ability to hide as a bonus action). Done.
Goblins, shadow demons and a few other critters come with this out of the box.
That way they can turn invisible and attempt to Hide straight away (instead of remaining exposed to attack for a turn).
Not that you need it with something like a Pixie. On thier turn, they cast invisibility (somehow) and fly up. Being invisible they are immune to AoO's as they move. Your PCs now get one turn to hit them with ranged attacks (at disadvantage) but not targetted spells (they're invisible so they're immune to these) before the Pixies get to attempt to Hide on turn 2.
They get +7 to Stealth, so next turn they should be all but hidden when they take the Hide action.
Wrong. Hiding is an action, but moving silently is your movement. It does not require you to hide to use.
An example of hide is: combat starts, the rogue is feeling nervous out in the open, so he decides to hide behind a tree. The enemy saw the rogue running away, and was about to give chase until he lost sight of him in the tree line.
An example of the use of stealth and hide is: the enemy crashing through the trees looking for the rogue. He stops within 30ft. of where he is hiding, the rogue could sneak attack with his blow, but he's unsure if he can kill this guy, and don't want the guy in his face the next turn, so he makes a stealth check (vs the enemy's perception at disadvantage) to see he can move away another 15ft before firing. He makes it, moves away, and fires with sneak attack, then uses cunning action to hide again.
What happens is the enemy get shot with the arrow and is severely wounded, he looks around in the direction of the arrow, but still can't see the rogue. Had the rogue failed his either hide or his stealth check, he would've lost sneak attack. Had the rogue been in melee range, he could've stabbed the guy, used cunning action to hide again, and then snuck away, had he failed his stealth check while trying to sneak away, his position would've been exposed in the movement, not the hide.
An example of stealth without hide is: you have to cross an open room, and the guard has his back to you. You make a stealth roll to get across the room without alerting him. The difference here is, there's no where to hide, so you can't use the action to hide if you fail your stealth check. (The guard hears a noise, turns around and sees you.)
As a DM, I would rule that an invisible creature is automatically hidden. It can move at full speed and make a stealth check against your passive perception at disadvantage. If he tries to move stealthily (at half movement), he gains advantage to the roll. The difference between a hidden rogue, and an invisible creature is, you have a chance of seeing the rogue and you can't with the invisible creature.
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