The Grell.. the scary brain beast with a beak and can paralyze you with a touch .. has been killing parties and floating away with their bodies since first edition.  But it has come a long way from ….
           To what it is today … LOL

That’s right this thing has been killing players since before it was cool. And its an efficient death machine. So just how does a Grell do its work. It starts the task with levitation. Hiding high up and out of sight this creature is an opportunist. I say this from experience. Way back in 1st ed My wizard had just begun to reach 8th level. I was coming into my own as a caster and on my rise to greatness.. And then it happened. I got Grelled. Our party was delving in a cave system( I forget why) and I was bringing up the rear of the group as my spells were running low. And out of the blue the DM says to me make a save vs. paralyzing. I failed and my party was unaware as i was scooped up in the tentacled embrace of the grell as it drifted off to feast on my wizard.
When it comes to the Grell you must remember this is not a stand up fight monsters. It has no desire to fight your whole party. It is just taking advantage of a good meal and your players are that meal. He just looks to drift on down from his position of safety and pick the choicest selection of meat, wrap it up in its gentle embrace and take his time enjoying the rare treat that is you.   And if your players try to attack it after it has his meal in its embrace the Grell is more than happy to take the order to go. One player will do, unless you have a few small races and in that case he will take a meal for now and one for later.
The Grell is not a stand up fighter it relies on its poison to do the its work for it. It has a terrible beak for snapping bone and crushing armor but let’s be honest. When you look like your brain is exposed to the elements. Your do NOT want to take a sword shot to the frontal lobe.
All that said, the Grell has not changed much from edition to edition. Because when used to its full deadly potential it is a terrible killing machine.  Ideally for a Grell the perfect combat encounter would be to quietly drift down sneak attack some poor player and silently drift back to its roost for a meal of paralyzed living prey. Leaving his fellow adventures searching for their companion. until the echos of snapping bone and tearing flesh gives away their companions fate.
But then who am I to know…I have only had it happen to my wizard. Damn you Grell ! Welcome to 5E may you continue to feast on players for years to come.

Suggested encounter location.
Anywhere the party is at a natural disadvantage to combat the Grell more than one or two at a time. Narrow bridges, Ledges along drop offs, Climbing ropes or tight corridors. After the initial encounter if the Grell has Paralyzed his prey move to keep few or no one in combat with it. Hovering out over said ledge or drifting up to the far wall out of range of the party to dine in peace on a ledge. 
I grabbed a this image. It gave me a grin because I can see at least 4 places where a Grell attack would be perfect.