okies - yes to all these. acid immunity dropped ..intrigued by variable might score.
Ok, I'll have a go at writing it up…
Waxing Might. The power of a gray sqaarg's attacks increase as enemies fight it. This power is measured by the sqaarg's Might score. The gray sqaarg starts with a Might of 0 and can increase it to a maximum of Might 8. For each creature that fights the sqaarg, the gray sqaarg gains Might equal to the highest ability bonus modifier that creature used in an attack against the sqaarg (including melee attacks, ranged attacks, or spells) with a minimum increase of 1 point of Might if the creature's attacks do not have positive ability modifiers. If the creature's attacks do not employ an ability score or its ability is undefined, use the Strength modifier of the creature.
A gray sqaarg's Might score resets to 0 if no creature attacks the sqaarg for 10 minutes.
Example: if a druid with STR 10 (+0), WIS 15 (+2) attacked a gray sqaarg with a quarterstaff and thunderwave spell, the sqaarg gains Might 2 from the +2 ability modifier of the druid's Wisdom spellcasting ability. Two goblins with STR 8 (–1) then join the fray; each adds 1 Might to the sqaarg for a total of Might 4. If another gray sqaarg then grapples the example sqaarg, it adds its STR 20's +5 ability bonus and boosts the example gray sqaarg to its maximum Might of 8.
EDIT: and here's some attacks to go with it:
Hold. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: If the target is a creature, it is grappled (escape DC 10 plus the sqaarg's Might score). Until this grapple ends, the target is restrained and the sqaarg can't make melee attacks on another target.
Crush. The gray sqaarg crushes a creature it is grappling. The target must make a Strength or Constitution saving throw (DC 10 plus the sqaarg's Might score), taking ## to ## (4d6 plus twice the sqaarg's Might score)[?] bludgeoning damage on a failed save, or half as much damage on a successful one.
Seize. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: ## to ## (2d6 plus the sqaarg's Might score)[?] bludgeoning damage. If the target is a creature, it is grappled (escape DC 10 plus Might). Until this grapple ends, the target is restrained and the sqaarg can't make melee attacks on another target.
I think that approach works but we might have to monkey around with the damage a bit to get the desired CR. I'm assuming we're aiming for Challenge 5 or thereabouts.
Might as well post a Working Draft too while I'm here…