Each sub section of missions requires you to beat them all on Veteran difficulty. There is really no easy way of doing this but you will need a lot of persistence, patience and reloading of checkpoints.
Some Tips that may help :
* Good use of smoke grenades is a vital part of the game, use them well and in the right places and it will make this a hell of a lot easier. Remember not to waste them though, as smoke grenades are not in plentiful supply.
* Always stay behind cover - try not to get stranded in the open or the enemy will often take you down. When reloading always do it behind cover as getting caught mid- reload will often end up with you dying.
* Always make good use of your fellow commerades. They will often be a great help and take out a number of enemies for you. They will not do all the fighting for you and you will be kept very busy, but staying behind while your fellow country men storm a house will often save you dying instead.
* When you get shot, as soon as the screen flashes red, get to cover as quickly as possible. This flashing red screen means if your shot again you will die. Stay in cover until the screen returns to normal.
* Always keep a careful eye on the grenade indicator, as soon as it pops up, be somewhere else.
* Choose your weapons carefully, when fighting in close proximity to the enemy an automatic weapon is always the best choice. A rifle is the best choice for the enemies at long range. Be sure to carry one machine gun and one mid to long range rifle at all times.
* When clearing houses of the enemy, grenades are the weapon of choice. Remember they will bounce off walls if you want to clear the enemies around corners without the risk of being shot.
* If your stuck on a certain mission for a long time, try a different strategy, head a different way, or try something different to what you have been doing previously. Different strategies will work for different people and the a various ways around many of the obstacles.