The second syllable: bee-LEEF.
It's the first 'A', = átmosphere...
It can be pronounced two ways. 1) ADvertise is the way I pronounce it. 2) I've never heard it pronounced adVERtise. 3) Though some people do put a bit of emphasis on the last syllable with adverTISE...
If you are asking about the emphasis, flounder has a stronger first syllable.