In the last installment of the Arkham trilogy we find out that the mysterious mercenary leader with an army who calls himself Arkham Knight is no other than
Jason Todd - previous Robin captured, tortured and presumably killed by Joker
That would explain his training, abilities and knowledge of Batman, but it doesn't explain where he got such a huge army with several hundreds of drone tanks and helicopters.
So where did he get the money / manpower to launch a full scale attack?