I was stuck on this for a while, too.
The obvious strategy does not include you getting out of that position alive on hard difficulty, so you need to pull back.
If you stay on the bridge, you are either gonna get shot, or blown up by the next rpg impact, which was my demise 80% of the time, as I lay prone, hoping I would not be seen immediately.
As soon as you grab the LMG, go to the stairs to the left where you climbed up the overpass and position yourself so, that you cover the pillars while looking at 2 o'clock facing the enemy. You should have no problem suppressing the enemy forces which get too close. You also get a lot of help from your squadron.
Apart from that, I got occasionally a nade lobbed over in my direction, which was easy to evade.