Detour
Advance To The Center Span
Don't worry about Advisor's constant harrassment regarding your speed of movement; this isn't a timed mission, a la the final mission aboard the Coreship. Proceed at your own pace.
To begin with here, take up the mine in your path, then kill all of the Trandos that come in before you proceed up the ramp nearby. There are a couple of snipe spots in the windows here, but you'll only be able to use one at a time. When you reach the second one, place someone there and keep them there while you take the rest of the team across the bridge and breach the doorway there. Kill everyone inside, then heal your guys up before moving down the ramp here.
In the next little area, you'll have two sniping spots to set up in, at the bottom of the groove that runs along the middle of the bridge. Do so, eliminate all of the enemies, then heal your guys up before you move to the end of the level.
A Bridge Too Far
Reach The Bridge
Hope you're ready for some fun, because the fight to secure the bridge span is easily going to be the most difficult part of the game so far. You have to proceed across the bridge and take down two separate SBD dispensers before you're able to even place the charge. It's gonna get messy!
At the outset, you'll need to move out to the span and take down the forces that are already on the bridge, which includes three SBD's. Place one of your soldiers in the snipe spot here, but hold the other two back behind him with a Secure Position command. These enemies are their own separate wave, so after they're dead, you'll be able to heal your team up without fear of reprisals.
The next groups of enemies are both on triggers. If you start slowly moving down the ramp, you'll notice that the door on the far side will open up, causing a number of normal battle droids to spill out, and that more battle droids will start using the turret off to the left, near the SBD dispensers (which are thankfully dormant at this point in the fight). The key here is that the trigger that spawns the turret gunners is a bit further down the bridge than the door-opening trigger, meaning that you should be able to walk forward until the door opens, backtrack and take down the droids, then walk forward again and trigger the turret gunner spawns, then use your sniper rifle to deal with that threat.
There are three turret gunner droids in all; one will spawn after the first one is killed, then the last will spawn after the second one goes down. By far the easiest way to deal with them is to snipe them in the head when they're in position, but you might want to be behind another commando during this exchange, as the laser fire from the turret is incredibly damaging, especially when you're playing on hard mode.
Destroy the SBD Dispensers
After the third gunner goes down, though, the fun is just beginning, as each SBD dispenser will begin spawning its troops. As mentioned, you'll need to take them both out before you can deal with the charge-setting for the bridge itself. Luckily, there are a couple of boxes of EC grenades to work with, as well as three or four boxes of anti-armor ammo, so you won't be hurting for tools. Your first objective is to blow the dispenser on the right side of the bridge, but you'll need to be quite coordinated for this to work.
What you'll want to do is move your squad up the bridge during one of the lulls in the fighting, which will occur every once in a while due to the way the dispensers pause occasionally. Since you're going to be taking out the rightmost dispenser first, you'll want to have your troops in position to cover the soldier doing the charge-laying, meaning that you'll probably want to set one up in the turret, with another in the anti-armor position on the right side of the bridge. This position has better sightlines to the left dispenser, which will hopefully prevent the soldier there from firing on the right dispenser, which causes significant collateral damage to the chargelayer. The third soldier is obviously going to lay the charge, while you should take up a position near the anti-armor soldier with EC grenades and an anti-armor launcher of your own.
The turreteer here will be taking a lot of fire, so you'll want to get that charge laid before he dies. To that end, protect the chargelayer with EC grenades (recall that the timer still ticks down even if the chargelayer is stunned by an EC grenade), but keep an eye on the leftmost dispenser and take down any droids that aren't being fired on with your anti-armor launcher. The chargelayer has a better than average chance of getting killed, though. If there's only a few seconds left on the timer when this occurs, you can run up and finish it off yourself, or just retask the anti-armor soldier to finish the job.
After the first dispenser is down, you'll want to order a general retreat. Revive any wounded soldiers, if possible, then get them all back to the opposite side of the bridge and start healing them. On your way back across the bridge, try to restock your anti-armor shells and EC grenades, if possible. A single soldier in the sniping spot here can hold off the oncoming SBD's while you take turns healing up.
Video Walk-through
The fight here can be quite tough, but if you go in and take advantage of the positions available to your squad, you'll live.
When you're ready to get back down there and destroy the second dispenser, start moving your squaddies back down the bridge, destroying the SBD's that have spawned in your absence. Now, it may be unorthodox, but we like to take out the second dispenser the old fashioned way. You'll notice that there's an anti-armor position near the dispenser that you've already destroyed; if you position soldiers there, in the anti-armor position you had them in before, and in the turret, then they'll continue to fire on the dispenser until it's destroyed, with no charge required. This will take a minute or two, but you're going to have some downtime after this fight, so even if the turret gunner gets killed, you're going to be able to heal them back up again after the dispenser goes up in smoke. Of course, you can also attempt to place a charge on the dispenser, but this is difficult to do while covering it with anti-armor.