Post's like these are a welcome break from the norm.
1) Helms Deep Put the best archers (i.e. the Elves) Up in the main hornburg. Break the causeway (It should be a drawbridge anyway) so the Orc's can't ram the gate. Put the Rohirim in the Hornburg, and order the outlet to stuffed with large boulders. The grate should be on the outside anyway, so they can't get charges within the walls. If they can't do that, then the blast will not be half as effective, I doubt it would breach the walls and even if it did crack them, It would still leave a large pile of rubble the defend. I think the Cave entrance is via the Main hall in the movie, so thats fine and dandy.
If the lower wall is breached (doubtful) or simply overwhelmed, then immediately withdraw all forces back up that very steep, narrow, exposed path. Pick the hell out of them with arrows. Wait for Dawn and the charge of the Rohirim.
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2) Osgilitath Keep the orcs in the water, fight in a shield wall. From what I actually know of the battle, I think its pretty much un-winnable with the resources available to Gondor. Mordor can just throw to many troops at the defenders.
However, I think the tactic of hiding and letting the orcs run past, then jumping out and stabbing a few in the back is a pretty damn retarded tactic. Complete loss of any semblance of unit formation, and it devolves into little more than crowds of troops running around in circles with no clue how many orcs are within the city and how many are on the lake. Meanwhile keeping them at the waterline allows the Men of Gondor to fight with secure flanks and rears, shields forward and with the orcs having to wade around. As the bodies mount it just adds another barrier to the enemy. If archers (those Rangers were good shots) could shoot the Orcs crossing the River, it just makes it easier. The Bridge crossing happened late in the battle, so if there Gondor has any sort of engineering corps, put them to work destroying the bridge.
However, I think the best thing to do would be to recognise that the city is just facing to many orcs, trolls and fell beasts, and make plans for a orderly retreat. Those Men in Osgiliath will be much better put to use behind the high walls of Minas Tirith than in the rubble of Osgiliath, and so few made it back to the city. Make plans for the infantry to withdraw, then the Calvary. Its not worth loosing the entire garrison to just hold the city a few hours. In fact, If I could, I would just order the whole force to withdraw to Minas Tirith right after I take charge, leaving the Rangers and a Calvary force to perform Harassment and rearguard to the Orcs before withdrawing. That way, the Men at Osgiliath are on the Walls of Minas Tirith, not being hurled over in catapults.
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3) Minas Tirith - I know Rohan is coming. I Know Grond is coming (and even if I didn't the Gatehouse is the obvious weak point). Use some of the Masonry that the trebuchet were using to make some rudimentary Tank Traps around the City - slowing the progress of Grond and the Siege Towers. Most importantly, Tie Denathor up in his bedroom and put pippin on guard duty, and Don't send the cavalry out to battle. If possible, pill rocks up against the base of the Main Gate, to hamper them if they break through (and with Trolls, I think that eventually they will might Grond up, though with a huge delay and greater cost and risk. Remember, they didn't breach the gate until it was almost morning, and the sun had dawned by the time they managed to force a withdrawal from the Lower level. Time is critical here.
Also, use the men saved from the safe withdrawal from Osgiliath to reinforce the garrison, and put the Rangers in position on the third level, with orders to constantly shoot from windows and cover, at the Fell Beasts, keeping them angry and annoyed. Maybe put a few on duty to try and snipe out orc commanders. Given the unrealistic accuracy that Faramir has (a shadow of Legolas, but still damn good), I think it IS plausible that he might be able to tag some foes. Either way, he will be a beacon for the Men around him, a force multiplier, which is probably more important.
The Men on the walls will be pre-emptively ordered to shoot the trolls. the Lower levels are evacuated immediately (Gandalf only orders them evacuated when the lower level was pretty much lost). The Aim is to survive the first day, and the first night and a little into the second day, for Rohan's arrival. Given that they managed that anyway, despite some cock ups, I think its almost certain that we will make it. Upon their arrival, once they sweep the front of the city, order any reserves I have left to push the orcs back. I do think that with good tactics, The Towers and Grond can be held back. If Grond can be destroyed by a trebuchet shot (it would only take one) then the Gates are not going to fall and neither will the lower ring.
Order the Knights (saved by not bashing their heads on Osgiliath) to ride out of the city and hack down any trolls and orcs at the base of the towers, then to fire them with Oil. The Rohirrim do NOT charge the Oliphants, instead withdrawing back to the north where they arrived, to rest their horses and re-organise. The Oliphants are useless against the city walls and can be outrun by the Rohirrim, so let the break on the city and get shot to shit by the archers. At this point, its only a delaying game while I wait for Aragorn to turn up with OP Army of the dead. At the end of that, I order the Army to go clear out Mordor.
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Edit in the last two soon.