You don't...
I've always heard that FA pharm is all you need to know, and while I don't completely agree, I think it's true for the most part. Sure, you will most likely get pharm questions on boards that you've never seen before -- that's just how the exam works. There's really no way to combat this, because you have no idea what those drugs are going to be. There are thousands of drugs in existence, and unless you plan on knowing them all, then you're just going to have to settle for the fact that you might run into one or two that you've never heard of.