I'd do the following:
GK: Defend (Nothing fancy here, Hart is solid)
FB: Attack (Provides width)
CB: Defend
CB: Defend
FB: Attack (Provides width)
CM: Defend
CM: Defend\Support (Depending on opposition, will still get forward though, regardless)
AMC: Advanced Playmaker Attack (Nasri)
AMC: IF Attack\Treq (Aguero)
AMC: Advanced Playmaker Attack (Silva)
ST: Whatever role suits Tevez\Dzeko
Play narrow, with a normalish tempo and high pressing. Make sure your fb's can get up and provide width, and your AMC's swap positions and run forward in and out of space.
I'd switch to a 4-5-1 with Milner in place of Nasri for the real tough games.