here's the rundown on that fight. To kill ram, you essentially have to play slow, don't run and gun it. here's my basic tactic I used when I went through the first time on co-op, although the same tactic could be used in 1p.
During the ramm fight, there are 3 major threats to deal with, the flying enemy's, the bats, and ramm himself. Ramm is only able to take damage when the bats are not swarming him. The bats do occasionally charge you, or your partner, when they do, make sure you get on a light, they'll disperse as soon as you do.
While there charging you, make sure you do as much damage to ramm as possible, I found that running him around the first barricade you take cover over (just hopping over it when he got to one side) and using a shotgun when hes open was the best tactic. if you have a partner, try to make sure he has a sniper rifle at the other end of the train (at the barricade under the mini gun), and he'll be able to take potshots at him while your holding his attention.
Don't use the mini gun, if the bats charge you, you have to move very fast to get to a nearby light, and if you don't get to a light that's a permadeath no res for you.
Continue running him around the barricade making sure your in the light when you hear the screeching, taking shots at him when hes open, and taking down the flyers in the downtime between bat charges. rinse, repeat, claim your achievement points.
let me know how this works out.