Since people are having a hard time using the Fountain Guards in battle.
Sword units such as the Gondor militia are basically able to fight in all directions. After the battle has started you could go around the 2 front lines that are fighting each other, and charge the enemy in the rear. If the enemy still has some reserve troops they will most likely charge the militia in the rear to deal with it. Because the militia are easily obtainable the initial damage does not matter that much. Also the last lines will probably turn around and fight that enemy.
The Fountain Guards are a whole different story. They are extremely hard to obtain and they are extremely immobile. There are 2 ways you can eliminate these weaknesses. The first one is by using the Fountain Guards in the front line, but a bit fallen back such as this:
Note that if the Rohan militia wanted to charge my FG, they would have no option but to charge fully into the line of my spears. The FG are in spearwall formation, the FG have very long spears so it useful to keep the enemies at bay.
The Fountain Guards have very high damage coupled with their AP. This means that you can also use them as shock troops. However as I already mentioned, the Fountain Guards are hard to obtain so it is important that you lose as few as possible. That's why when I use my FG as shock troops, I make sure that the AI cannot abuse the fact that they are sitting ducks during that time. I place my troops such as this:
Note that my Gondor Militia is keeping their front line in place. While I used my cavalry to keep their reserves in place. I maneuvered my FG around the front lines and they now have a clear path to charge the enemy in the back, while not having to worry at all about being charged in the back themselves. Also the Fountain Guard are NOT in spearwall formation, this is because I want to be able to get the charge bonus.
Of course I only used a couple of units in both of these battles for clarity, but my examples are usable in large scale. Good luck with your Gondor campaigns!