"Poof!" That's the word that really sums up the extinction of all life on the planet, isn't it? You got your BANG and your KABOOM of the earth-shattering variety, but people really life in fear of the day things go POOF.
Let’s consider the logic of the villain here: he has a force-field capable of encasing the entire planet. He has a bomb which will destroy all life, for whatever reason he has (and I’m sure they’re solid and logical.) Means and opportunity, then. What’s the next step?
Of course: attach the explosive device to the most powerful creature on the planet. Not put it somewhere he’d never find it; not keep him busy with a distraction. No: put the bomb on his belt, give him the motivation he needs to defeat you, then set back and watch all your plans go to hell. Nice work.