I have not played on pro mode, but I'm going to assume that the difference is health, and that a tactic which makes it easy on normal mode is going to help on pro mode as well.
1) (You maybe know about this trick)
On Mendez' second form (where you start the battle), immediately turn around and climb the ladder behind you. Then turn left and run the ledge towards the other ladder. Stop behind the structure and stay close to the wall. You can shoot him through the structure thing, do that. Also make use of that gas barrel nearby, lure him close to it and make it explode. If he survives, return to the first position and shoot him more.
He can not reach you if you stay behind the "structure" and near the wall. Because you look down on him, it's easy to hit his plaga tentacles. I believe that's a vulnerable part. His spine is also vulnerable, but difficult to hit from up there.
Use any of your guns here, but save your shotgun shells till later. If I recall correctly, the distance is a bit long for a shotgun.
2) When you fight against Mendez' torso (I call this the third form), you can stay up there, but you'll have to run around. Make him fall down (either with your shotgun or a flashbang), then throw real grenades on him. I advise doing this on the ledge, because the grenade blast will really hurt him even if he jumps up before the grenade explodes. If you do this down on the floor, it's easier to miss with the grenade.