See, and IMO, I think her intelligence would matter quite a bit, because it opens the door to much greater tactical thinking. For example (and this is just a brief easy example off the top of my head), since we know she's usually around other demons, she would instruct them on tactics they normally aren't smart enough to think of on their own. Like telling two barlgura's to keep invisible until the party is within sight. Then have them leap and grapple the fighter types (maybe tossing them into lava pits, or even sacrificing themselves to bring the PC over the ravine along with them, etc) as the dretch pour out of hiding to act as minions that would have lots of attacks of opportunity if the PCs tried to move around and engage the marilith. Meanwhile, the marilith uses teleport to go right to the casters, making mincemeat of them with her grapple and 7 attacks. Her parry reaction (which can be used every turn) would give her an AC of 23, which is still pretty good even against level 16 PC attackers (that would have an average of +10 to hit assuming prof bonus and max attribute bonus). She would keep teleporting out of range of the PC melee types, who have the choice to keep chasing her at the cost of all those dretch AoO (assuming they've managed to break free of the grapping), or spend several rounds at the very least taking care of the minions, all the while she's in melee range grappling the squishy casters. A marilith has the potential to take out a wizard in one round. Even with rough averages, it would only take 2 rounds.
PLEASE Sacrosanct - you
cannot include other monsters in the Marilith's Challenge Rating.
And your teleport tactic would be GREAT. Except she doesn't get Teleport as a bonus action. She can't dish out ANY damage in the round where she teleports. She teleports to the Wizard. The Wizard teleports away. Now its her turn again.
She can't teleport out of range of melee types, unless that is all she want to do in the fight. Note she has zero ability to deal damage at range. That's always a critical weakness, but is she given ANY tools to make sure she gets to dish out her signature melee damage? Nope. None. (Unless you again bring in adds)
Her Parry is only good against ONE attack per turn. While this is far from bad in general, it does her absolutely no good against a party fighter that decides he's had enough of her. As long as the party is level 11 (a fair assumption), she can only block
one out of six attacks. (Yes, I'm talking about Action Surge. And at level 15, for a sufficiently minmaxed fighter, that number could be
one out of nine).
I can understand she's very dangerous in your dream scenario, Sanct, but to make her stat block actually support that scenario, I need to do the designer's job for them. Try these quick fixes:
Teleport
(recharge 5-6) The marilith uses her
bonus action to magically teleport, along with any
equipment
or creature it is wearing or carrying
or grappling, up to 120 feet to an
unoccupied space it can see.
That change alone means a huge jump in actual challenge. Want more. Let's suggest we add back just one of three of the things she needs from the work of previous, more competent, designers: Mislead, Crishing Coils, and Unholy Aura.
Let me take you through each of them, step by step:
Mislead as per the 5e spell, allowing her to sneak up on the party while sending their melee types on a wild goose chase. Even a single round is gold in high-level combats. This spell, even if she only gets 1/day does wonders to enhance her reputation as a
snaky sneaky bitch
Crushing Coils (Ex)
A creature that takes damage from a marilith’s constrict attack must succeed on a DC 25 Fortitude save or lose consciousness for 1d8 rounds. The save DC is Strength-based.
This is Pathfinder, so don't look too closely at the mechanics and numbers, but you get the idea - if she can start taking people OUT of the fight, she's that much closer to earning her status as described.
A third suggestion would be to give her (back) the ability to cast
http://www.d20srd.org/srd/spells/unholyAura.htm
This is because every objective analyst of the 5e Marilith stat block immediately notices a complete lack of a buffing capability. If she can't enhance her troops, what kind of leader is she?
Are you really so blinded to your desire that 5th edition be flawless that you truly can't see the objective flaws and weaknesses in her design...?