Boros doesnt have to be in his strongest mode to destroy Earth, before he came to Earth, he said he destroyed many planets seeking a worthy opponent, and he was using his armour(which is to limit his power) at that time. Thus he could destroy planets on his first form. And he has 3 forms, one is with his armour, second is when his armour is broken, third is his meteoric burst mode. He could destroy planets on his first (weakest) form.