Assault boats, eh? I thought however that necrons got their reactive hull save against Hit and Run attacks as well as repairing critical hits on a 4+. Not to question your judgment, rather just out of curiosity, how does that work?
They don't get their reactive hull save (5+, 4+, whatever) because that is considered a shield and a-boats ignore shields. They do however get their 2+ save when on BFI, because that's a brace save. This is the same ruling as for the Warp Cannon on an activated Blackstone Fortress. [...] [Warp Cannon ignore holofields for some reason, even though they're a type of lance and HFs are meant to spoof lances, but hey, it's GW, what can you do? Who plays the official Eldar rules anyway?]
One way to counter necrons is to make sure that the player plays by the rules, so I figured I'll ask for a clarification.
I'm wondering what the 4+, 5+, and 6+ save (and the 2+ save) does or doesn't work against.
From what you're saying the 4+, 5+, and 6+ save is a shield, and so it protects against guns, most lances, and exploding ships.
Gauss Particle Whips that roll a 6 ignore shields, so would therefore ignore the save in necron vs necron battles. Tyranid Bio-Plasma and Warp Cannons are lances that ignore shields. Ordnance, boarding, ramming and fire ships all ignore shields. These therefore ignore the 4+, 5+, and 6+ saves.
The save is 6+ for Raiders, 5+ for harvesters and Scythes and 4+ for tombships. If the Necron vessel uses Brace for Impact then the save is modified to 2+
If we're interpreting "then
the save is modified to 2+" as "then the
Brace for Impact save is modified to 2+" (rather than "the
reactive hull save is modified to 2+") then the 2+ save works against everything except for boarding.
How much of this is right?