It isn't to hard, and the other 2 people can die.
Only YOU have to not die during the match. So just be careful, and if you really need to have friends help you. Have them use medic or protector, and just don't rush out acting rambo. If you really need so, you could equip juggernaut or medic yourself, juggernaut being extremely useful as you can't die while this is activated.
regeneration as a slot 1 booster shall help as well.
Remember, the other 2 guys can die, to get the trophy, YOU have to not die the whole game. Just play cautiously really and if you feel you're overwhelmed, then retreat.
I can confirm this also…the trophy is individual. When we first went for this, we were trying to keep everyone alive until the end. At one point, I think one of us died near the end of Round 10 and we decided to finish off anyway for the cash. Lo and behold, two of us got the trophy while the dearly departed did not. We simply tried again while moving heaven and earth to keep the one “Invincible”-less player alive until the end.
Some of the trophy descriptions are poorly worded or easy to misinterpret. Another example is the Airport Trophies. I believe killing Eddie, Flynn and Laz have to be within the time limit of each other…so instead of killing them ALL within 20 seconds…you just can’t have more than a 20 second span between their deaths. So in theory, you could kill Eddie then Flynn after nearly 20 seconds and then another 20 seconds till Laz.
One more thing…the Hard and Crushing trophies are cumulative (like completing the campaign). So if you achieve a Crushing trophy, you should automatically get the Hard one as well.