@tim:
A potential justification for the rampant one-shotting: the doflamingo pirates have gotten old, fat, lazy, and overconfident. That was the point of laws speech to doflami go on punk hazard. The doflamingo pirates have just been sitting on their hands while the new generation has been training and getting stronger; this is particularly visible when you compare the flashback doflamingo family to the present day family (notice how machine, jiola, and señor pink in particular where much more toned and combat ready).
I think this is the first arc I've felt that the Strawhats were really just going in for assistance and not being at the fore front.
I think these 2 points go a long way to explaining why the DD family have been underwhelming.. It does help to have one of the strongest Shichibukai as your captain. His reputation alone is good enough to ensure almost nobody challenges them.