Clash of the Titans (Secret ship spoilers)

[Warhawx]

The achievement for the Crystal ship, which requires you to kill 15 rock ships- must I do it in a single playthrough or can I kill 15 rock ships across different playthroughs as the crystal ship?

[Leezeebub]

All other cheevos are for a single play through, so I would assume this is the same.

Just need to be lucky enough to have a few rock sectors in a path.

[Oka]

killing crew only works or must destroy the ships ?

[breadsmith]

I'm pretty sure you need to destroy the ship itself, not just the crew.