The only character that really fits that description is Thea who suddenly became a ****** after a mere five months of training with Malcolm (while holding down a job, no less). Maybe Roy, but even if he didn't get formal training until recently he grew up fighting anything that moved in The Glades and they've shown him make enough mistakes as Arsenal to show that he isn't quite capable of running with the big dogs on his own just yet.
Everyone else has years and years of training at the hands of very dangerous people.