I use a pair of OXO kitchen shears and it goes through the clam shells very easily. They combine good leverage, very sharp blades, and just a bit of serration.

I use a pair of OXO kitchen shears and it goes through the clam shells very easily. They combine good leverage, very sharp blades, and just a bit of serration. With any other tool I've struggled as you have, but these make it pretty straightforward to open them.