ok..how to Save them, everytime i play that Quest that Colvin,Vargas and few others die while fighting the Enclave in the bridge...

I tried everything :\ except , using tgm and killing everyone and everything with 1 shot....

Think of it as your opportunity to pick up more gear.... I go to that battle in my underwear and no weapons, in order to pick up more gear. I've never been able to save them. I've certainly tried though.

You can run ahead of them and do all of the fighting yourself and save them. It helps to have Fawkes as a follower and taking Charon along outfitted in the Chinese Stealth Armor with a sniper rifle would be a good idea. A one-shot kill weapon such as A3-21's Plasma Rifle or Lincoln's Repeater is your best bet to keep them all alive.
If you find Dad on your own before going to GNR, Paladin Vargas will be essential during Take It Back so you wouldn't have to worry about him.

That being said, I always use the same tactic as mpls Big Daddy. Their loss=my gain.

open the console click on the dead body and type resurrect

Well since you are using console commands why don't just use resurrect on the members of the pride that were killed?

go and do the battle again and take a mental picture of where all the baddies are standing. the reload and before the battle go and place a bunch of bottle cap mines at the places where the baddies (will be) are at . then start the quest. (now try not to put to many mines to make sure you dont kill any BoS by accident) NOTE it will work as long as the DEVs used the same instance . it has worked in other places . Like the raiders that always spawn by th outcasts hidout . after kiling them place a bunch or frag mines close the where the dead bodys are . ANd the next time you fast travel to the outcasts the raiders will auto spawn and start setting off the mines. its funny as hell