The issue with biological warfare is that both we and the aliens are going to have to basically rediscover the entire field of biology from first principles before we can even comprehend each other.
Their brains are going to have to operate on some system entirely different from ours, you simply can't make a structure of neurons the size of a pinhead and get enough complexity for a human-level intelligence. So whatever they're running on isn't made of carbon-based cells.
Meanwhile they've never encountered anything as large as humans, indicating that whatever weird biology the rest of the universe operates on, it apparently does not allow for anything as big as a human, much less a dinosaur or whale. Now bigger isn't always a biological advantage but generally for at least the apex predators, large is an advantage so their biology must somehow quit working if their atoms combine into a structure the size of a human.
Thus fundamentally we're both going to have no idea how the other's biology works. They're probably not even made of anything resembling cells and won't have any idea how to engineer a cell or attack cells. Meanwhile we'll have no idea how to engineer a disease that isn't either a cell or infiltrates cells so our best biotech will do nothing.
Squashing, however, will still work unless they're made of some exotic matter instead of baryons.
