I'd say that hitting a stationary object provides a +30 bonus, on top of a -10 penalty because the target is smaller than human sized.  The potentia coil has 15 structure points, and an armor rating of 3.  To create a "safe zone" you'll have to shut down two coils, and then you can move about by destroying more coils.  There aren't any coils in the central reactor core, which is where Harlock is.