I was thinking of putting Giga Impact on my Choice Scarfed Braviary, but if I can't switch out on the "must recharge" turn, my Braviary is dead meat. Please help (and suggest a move over Giga Impact mabye).
No you Cant
You are forced to stay in on the recharge turn.
As for another move I would suggest Crush Claw since it has STAB and a 50% chance to Lower Defenses.
Braviary @choice Scarf
Defiant/ Sheer Force
252 atk 252 speed 4 hp
Crush Claw/Return (Sheer Force Crush Claw does about the same as Return But if defiant Return > Crush Claw)
To put it simply, no you can't switch out. However, if the opponent uses a move such as Dragon Tail on the same turn, you can.
A good move to replace Giga Impact would be Return or Secret Power.