heat the BB with a portable propane or MAP gas torch.
If this doesn't work, dynamite.

I had a hell of a time with a Shimano BB and an AL Diamondback frame. It was left submerged in water.
Took a PB Blaster soaking overnight, followed by clamping a Park tool to the cup and and a super long breaker bar. Had to stomp on it with my right leg. But it finally loosened.
Don't know why people insist on not using anti-seize compound for dissimilar metal contacts.