ac4 is terribly optimized for crossfire, the only thing you can really do is drop the settings to the floor and possibly try to lower the resolution. it is a demanding game that
1.favors nvidia
2. is terribly optimized and fairly cpu reliant.
also, your system ram is the clear holdback of this system. ac4 requires at least 4gb, and for an apu, you'd want even more (speed of the ram also becomes important when dealing with an apu). (i'm willing to bet your ram is lower than 1333mhz)
basically you need to upgrade your system a bit