How do I make perfect/critical shots?

Right next to the cannon, every drone has a blue glowing battery-like thingy. You need to shoot this with your vulcan canon (the machine gun on your bat mobile). To perform two in the row, you need to destroy two drones like this in quick succession without getting hit (like getting combo meter up while brawling)

Aim at the glowing blue battery present on most tanks. Shooting it out will cause an instant-kill.

Hitting these glowing batteries counts as a critical shot.

Hitting these glowing batteries without making any other shot on the enemy counts as a perfect shot.

You can pretty much do this from the start, though Oracle will update you with the intel regarding the weak spot roughky a quarter to half way through the game.