As both AlanSE and Philipp said, Pu-238 won't go boom no matter what you do to them.  However, lets pretend it was Pu-239 and see what happens.

First, the battery put out only about 1/2% of the power of a Pu-238 battery, Galileo doesn't work at all.

Second, while it is just barely possible to make a mass supercritical with pressure you would have to start with a mass that's just a hair below critical and then subject it to some really extreme pressure.  While we thing of metal as incompressible it will squash a bit under planetary core type pressures.

Finally, if you manage to do this you end up with a criticality accident, not a mushroom cloud.  The one I recall reading about knocked a nearby worker off a catwalk--he lived long enough to run out of the building.  If you want a boom you have to convert the subcritical mass to a very supercritical mass very quickly--which is why all the complex mess with explosives and very precise detonators.