I completed the campaign on veteran the other day. For me, it was more a game of patience and perseverance than actual tactics.
As for the timed submarine, that was one of the hardest parts for me. I eventually did it through a lot trial and error, I will list what I can remember regarding how I eventually did it:
- 1st predator missile, take out the helicopter on the landing pad. That way you can hide behind cover to heal without having to worry about being hit from above.
-Try and save your predator missiles for the troop transports and other enemy vehicles, but at the same time take out as many foot troops as possible in the explosions. Also, dont waste time and missiles trying to shoot enemies that are not directly on or blocking your path to the sub.
- Push forward as quickly as possible while making sure you are thorough in removing all enemies on your path to the sub. There was nothing more annoying that making good time only to be shot in the back by someone you missed.
- Although they get in the way, try and stick with your team mates as much as possible, they are not completely useless in veteran and can help in drawing fire away from you.
- Sprint to the end when you are near enough. I made it with about two seconds to spare and died a split second after reaching the check point.
- A little luck!
As I said above, trial and error. What worked for me may not suit your play style so just keep at it and good luck =D