Batman had been pursued the moment he charged through the space portal.
The Chitauri fleet hadn't cared about him at all at first. They continued to send troops into New York and had a small squad go after him.
Of course, this small squad itself already had more than 500 fliers.
Luke could only keep up his speed and stick to the big battleship as much as possible to block some of the attacks. At the same time, he relied on the fact that his clone wasn't afraid of death to fight the fliers.
The Bat suit had an exclusive defensive component, which was the only one to use a third generation arc reactor. The suit hence had an extremely powerful energy system and protective armor.
