I'm thinking of leaving out the added complexity of captured Judgement Bringers ships, but the basic idea is this:
>Saboteurs are infiltrated aboard a number of cargo vessels.
>Warp drives are sabotaged to allow smaller Sky Serpents vessels to quietly infiltrate the Catachan system, perhaps over a matter of weeks
>The first wave of landings are covered by some cargo-ships going nuts in orbit, ramming docks and defense platforms. That's actually why I'm considering using captured destroyers-- they can tank more firepower and it's not like the Sky Serpents don't have a reputation for this sort of thing. The ramming docks is a bit unusual, though, since its basically suicide, and the Sky Serpents infiltrators tend to be quicker to cause some chaos when some place is so heavily defended. That and the Sky Serpents tend to bypass such strong points, leading some in the Arms to speculate that the true strike is coming elsewhere, leaving them focused on a warp-bourne assault on the auspex net.
>The sudden chain of explosions raises suspicions and security is tightened, but by this time, it's too late.
>Larger fleet enters the system after capturing a few vessels to lead them in.
>Traitor fleet makes for the mandiville point to repel the Sky Serpents invasion
>The ships that had been snuck into the system fire up the engines and begin their attack runs, even as forces prelanded begin assaulting their objectives
>Open a hole in the orbital net for the Sky Serpents to land more troops and suddenly there's a battle in the jungle, with the local wildlife going nuts from being creeped out by all the psykers.
>Then something else happens, not sure what, maybe the whole attack on the sensor net facility is a feint, they're actually going for the orbital defense net because Xun says fuck it, I'm not going to siege that place, it's got crazy defenses. Instead he knocks out the defense system and exterminatuses it...