You need the sniper and lancer for this.

Take cover behind the barrier with the sniper and watch him.
As soon as he enters the light, snipe him in the head continue this until he gets a few feet from the barrier.
Then switch to the lancer and blind fire him in the chest and head (try to get a perfect reload).
He has a habit of walking side to side trying to shoot you, so keep shooting and stay in the middle.
When he tries to walk around the barrier rush to the other side and dive out and run full pelt up the train to the second identical barrier (you will take minimal damage) and start the process again.

Using this technique I defeated him on hardcore difficulty.