As others say, the Marines get a 4+ save (unless they are scouts), and I would expect them to end up in B-B (since they would not be "disembarking" as such, which only occurs after a move).

At that point the HBs are likely to die in the normal round of fighting . . . .

As they would be expected for such an attack. Play with the bull, you get the horns

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but first, they have 3 FS attack 1 for the transport -if succesful, 2 against the guys inside. worth it/or die tryin

Not sure where you're getting that HB have Extra Attack +2 from (unless something seriously wrong has happened in Eldar land )

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+2 attack =the save of the guys inside the transport! one 2+ CC for the HB, means 1 save for the transport (in case of a Rhino is 5+) -if it misses, 2 saves for the guys inside! and it's free, they are auto hits!