So if you play with any LOS blocking terrain at all, like you should do, then you can replace the Grimoire or Portalglyph whenever it dies?
My mistake it is 3. But still the deamons most likely to survive a turn are bloodcrushers as they are T4 which is harder to kill or plaguebearers, for the same reason but the also have shrouded (though less offensive capability than bloodcrushers).
It also didn't help you had some uncanny rolling of 6s....
Tyranid Warriors with a 5++ and no armour save are not hard to kill. Hell, a single Devastator Squad with Missile Launchers will rip right through them. Or a Battle Cannon, or a squad of Meltas, or like a single Tactical Squad.