@ShenLong:
Blackbeard should be more powerful than he is in the moment, because the Yami Yami no Mi is said to be the most evil DF and now he has gained the Gura Gura no Mi's powers. But still he got easily defeated by Magellan and Whitebeard and ran away from Akainu.
Magellan and WB don't count, he just had his Yami Yami power… Magellan was the ultimate worst opponent for BB, 'cause if he sucks in poison, it'll work even better. DF work like the elements of Naruto. Strong against one, weak against another. And WB did do severe damage to BB, but couldn't kill him because the fight was stopped by BB's crew finishing WB of. No one knows what would have happend, if both were at full health batteling everything out.
And for Akainu: he didn't want to fight him yet (click), doesn't mean he had fear or thought of losing. He surely will fight him eventually, maybe after having full control over both his fruits. No one ever had two, so we don't know how long this might take…