In co-op get all the ammo before fighting Ramm.
One player stays behind the first initial barracade while the other player runs to the first crate on the left.
Then the first player keeps shooting Raam with the torque bow.
When Raam passes the second player you should run all the way to the end of the train and jump on the machine gun (watch out for Kryll though).
Shoot Raam full of bullets while the first player distacts him with the torque bow.
If the Kryll ever come after the second player, they need to get off the machine gun and hide in the light until they go back to Raam.