NPCs coming to save the PCs is simply part of the fun. To think that the PCs can do everything in a game and universe so fucking grimdark is stupid. Sure, you could give the acolytes a big fucking gun and point it at the daemonhost, but that is no fun at all.
In the pure mythos of 40k, the reality is that no five cells of acolytes are likely going to be able to take down a daemonhost of such stature. 1d10+34 damage is more than I think anything ever will take on. Think, for instance, of Chaerubal (or wtf idfk) from Eisenhorn, who FUCKING TOOK DOWN A FUCKING TITAN.
Then you start to see that perhaps calling in a regiment of Guard, a nearby contingent of SM, or even a 5-man of GK is not that bad of an idea. And besides, any reasonable acolytes are already gathering a fuckton of insanity points from just looking at the damn Fear4 beast, not to mention its lightning breath.
So do whatever you think you should do in regards to an actual Imperial reaction to this shit. It could just be, as previously stated, a well-equipped daemon hunting Inq. Read the DotDG for ideas or something, like Hexagrammatically warded weapons.