You actually don't.

Step on me snake mommy

… Smash.

think of it like having a reaction for every creature/player there is in the encounter, vs one reaction per round.

Where did all their spells go? They used to be able to cast Cloudkill and a lot of other cool spells. 

I was trying to homebrew Gherman the first hunter from bloodborne and didn't even realize that the marilith was basically what I was making 😅

Seems like an enemy where any kind of reaction suppression or attack reduction (such as the Slow Spell) has a chance to be highly effective in spite of the magic resistance, either way an enemy having 7 attacks is kind of funny

Not once in a round but on everyones turn... everytime its someones turn that thing can parry - thats crazy

Use ranged combat. Have MU's or clerics banish it. Stun it. Blind it. There are a range of options available. Personally, I would start with trying to blind it if possible. Diminishes the melee and completely negates the teleport ability; then go for attempting to stun it or grappling it. You could try polymorphing it into a lesser creature, like a chicken, tie it up, blind it, turn it back and it's essentially disabled. Not saying any of this is easy, but it all achievable.

Technically a PC could use 2 reaction in the same round.  For example, if the PC is in the middle of the turn order and a monster at the top of the turn order in round 2 decides to move outside a players melee range without first using disengage allowing the PC to use a reaction to take an attack of opportunity.  After doing so, play continues until it is the PC's turn who then gets his reaction back.  After completing his turn, another enemy combatant moves outside his melee range without disengaging and the PC has his reaction back and could thus do a 2nd opportunity attack on the same round (round 2).  Obviously, this hypothetical scenario assumes the PC on round 1 does not use his reaction on any subsequent turns to his own on the 1st round.  The only way dnd could ensure players only took one reaction per round like your comment suggests would be to change it so everyone gets their reactions replenished at the very end or start of a full combat round.

I found a video on how to run a Marilith on YouTube:

https://m.youtube.com/watch?v=dQw4w9WgXcQ

Woah, they got amped up since AD&D 1e. I'm porting Dragon Magzine #100 adventure "City Beyond The Gate" to non-optional core RAW Dnd 5e. The big bads are 2 Type V Demons - now renamed Marilith Fiends. Courtesy of the satanic scare over old D&D. Turns out one not two, by new CR, is the appropriate encounter. Glad I checked! Segue. I've been toying with a 'high fantasy' homebrew rule. It's just always roll 2d20, whether or not you have Advantage or Disadvantage. Take the most middling roll, rounded up. I.e. a fighter lord vs mook rolls a 3 and 12. 12 is more middling (10.5 is the average roll, so call it 11. So a 12 and 10 would round up t 12.) Why? Cuz a foe with good AC and to-hit roll mod would be HOSS. But also, name-level heroic party would wade thru mooks of any sort. You'd hafta reconsider the CR for it. I but I think my proposal would downplay the historic wargame simulationist aspect of the roll 1d20 to hit aspect of Dnd. And finally make it high fantasy...