Page 18, Intervening Models and the rule is clear.
"If a target is partially hidden from the firer's view by models from a third unit (models not from the firer's unit or from the target unit), it receives a 5+ cover save in the same way as if it was behind terrain."
A Monstrous Creatures is granted a 5+ cover save if it is 25% obscured by terrain. So then, we determine if the MC is 25% obscured by the intervening unit. If yes, then 5+ cover save. If no, then no cover save.