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Fingers crossed!

Considering the stated Ghostbuster's influence for the Slimer, I think this is appropriate:

Perhaps a bit too similar to (or basically what you're going for with) the Smokebomb and Replace enemy, but a changeling enemy that pretends to be one (or more) of the party, mimicking their moves somewhat incorrectly. At the start of battle, they cast an AoE on the party that (reduces the accuracy of/make them invulnerable to) all but one of the Princesses (whomever they are mimicking) due to the others uncertainty, until the enemy is beat or the spell is broken somehow. Perhaps doing some naughty stuff to try and 'awaken' memories, or 'love' based variants of the normal attacks.

Depending on complexity, you could have an encounter where a Replace changeling replaces one of the Princesses at the same time as the enemy "Pretender", leaving the player to battle the real Princess (distinguishable by a subtle difference in the sprites/attacks used, and/or immunity to certain effects). It would effectively be the same encounter, just have a hidden status effect on both the enemy and the princess to determine which sprites are used, and the outcome when one of them is defeated.

Might have to have a trigger for when the Replacer realizes they've been had though, and they turn on the Princesses.

Or instead of being the princesses themselves, fake love interests (or Shining Armor in Cadance's case) that pop up out of nowhere and try to gaslight the Princesses into believing they've been made to forget them. Desperation leading them to try and reignite their love with true love's kiss (and more). Trying to prove they're genuine by 'revealing' their target Princess is into certain kinks, and forcing them into it. The target would have to be determined at the start of the fight, or during the first round.

Alternatively, a trio of Changelings pretending to be the Princesses. Being defeated by themselves, or even the sex slave variants of themselves, would be humiliating.

Floor/wall traps that are green cocoons. One princess is partially restrained and/or stripped, while the other two have to deal with a small ambush. Could have an enemy type that initiates the animation mid combat, similarly to the rope enemies.

For the Changelings as a whole, you could have a theme of heart shaped aphrodisiacs/pheromones. Make Alicorn physiology resistant to the stuff, but leaving them briefly stunned (in lore) and with dulled senses. This plays into the existing Smokebomb idea, and could be the basis for the Pretender's initial AoE.

Building on that, could have an enemy type that plays similar to the Drone Tech, using pheromones for a similar effect to hypnosis. Plus, for more recycling, they could force an aphrodisiac filled gas mask onto one of the princesses for a variation of the Hypno Helmet animation.

Probably more work, but given that you have to take out the Slimer from behind, could be fun to have a still image of one of the princesses sending his equipment into overdrive, making him waste his shot in the wrong direction (possibly with friendly fire). Or if actually taking him down from behind, adding a lewd still/animation if there is a Replacer in the party, potentially even alerting the Slimer and betraying the Party to start a fight.