Incorporeal monster with spellcasting or spell-like abilities (ghost, spectre, shadow demon...) plus ooze (like gelatinous cube).
The spellcasting monster hides itself inside the ooze, shielding it from attacks (who's wanting to enter inside a gelcube to bash away at a spectre? and those area of effect spells won't reach the incorporeal baddy unless they completely toast off the ooze); letting it unleash its spells or SLA at the PCs to soften them for the ooze to swallow. Spells or SLA like hold person are especially cruel in that combo.