|I have no affiliation with the Southern California Adventure Racing Buddies ... but the organization's name is funny.|
Just a quick PSA that the probability is nearly 100% that any major FAQ and tournament out there is going to confirm all scarab swarms spawned in a given turn are spawned at the same time, and thus cannot be placed more than 2" from any scarab swarm base that was there at the beginning of the turn.
So, while some of you out there are laughing and playtesting away with the ability to create the Scarab conga line of doom ... it's not actually going to be something you can do at tournaments unless GW specifically FAQs that you CAN. That's to say - even if GW doesn't FAQ that question in its Necron FAQ, it will still be played per the above in any major GT you attend (INAT, NOVA FAQ, whatever) once all is said and done.
So ... I strongly encourage the pointless arguing to stop, and the playtesting and playing to occur at a more normal pace. It's just silliness.
Re: Entropic Strike timing ... I play it as it is written, which is to say the armor reduction is applied immediately following the hit (and therefore prior to the armor penetration roll, which is separate from the roll to hit). This makes Scarabs even more exceptional against vehicles of all sorts ... but freaking out over this isn't probably a necessary thing.
Necron actively struggle with ranged anti-tank, and Scarabs oddly enough ARE ranged anti-tank (in that they can rapidly and effectively disable or damage multiple tanks in the early phases of the game, in a way that does not expose critical units to counter-punches, and in a way that doesn't take until Turn 5 to be applied, a la Tyranid Monstrous Creatures). If you significantly reduce the effectiveness of Scarabs with overreactive FAQs or "homebrew" "this must be what they meant" playstyles, you'll quickly find Necron struggling against top tier "parking lot" armies. A handful of single shot Eldritch Lances walking around ... a couple of over-priced over-fragile Triarch Stalker shots and some AP- Annihilation Barge fire is going to kill a tank or three, but it's not going to level the playing field against 9-Chimera/3-Vendetta melta-spamming IG lists ... or Razorback and Long Fang spamming wolf lists. Necron need Scarabs to better build them for all comers win-loss tournament play. Blame the odd design choices in the rest of the Codex if you wish ... or lament the more mandatory nature of the unit ... but take care not to try and eliminate the only super-reliable option they have by over-home-FAQ'ing or over-complaining ... b/c you'll wind up with a lot of unhappy Necron players excoriated for "just taking scarabs like everyone else" or nerfed into Tyranid/Demon levels of Codex struggle against the broad field of an all-comers tournament.
Just my $.02