have you tried to hide behind checkpoint or loading area?
i have not tried heroic, but in Normal and Hard, enemy rarely chase you through a gate and never chase you through checkpoint or loading area.
EDIT:
they do still shooting at you, they just wont chase you.
On Heroic, a captain will consistently spawn in the hallway before the checkpoint to stop you from doing this. I think he is spawned on normal too, but is easy to deal with. On heroic, captains aren't as easy to deal with. It will typically take all 3 fire team members focusing target for a minute or so to bring one down.
You can hide on the sides, but mobs spawn and are more aggressive than normal and hard. Also note that not killing them and focusing on the boss is not really an option. I wasn't kidding when I said they will one shot you. On normal and hard, it doesn't matter so much if the occasional stray bullet hits you. They don't do any damage. On heroic, you notice every hit even when they don't kill you. If you have too many mobs alive then you end up getting hit by 2 at the same time and killed. We actually wiped twice when the boss had to have less than 10K hp left because a huge wave of monsters spawned and immediately launched mortars and grenades at the hallway and the side room where we were hiding - effectively killing all 3 of us instantaneously. In the end, the time we got him we saved our grenades and specials and when that wave spawned just dropped them all on top of them.
I'm sure there will be places you can find that you can cheese the encounter. Personally, I'm happy my group decided to just do it straight up instead of looking. It made the encounter feel far more epic.
I actually think the encounters were pretty well balanced. They required teamwork and some measure of skill. It made it a lot more fun to me. Normal and Hard you can really just zerg though. They become more of a contest to see who can kill the most than a teamwork exercise. I like that heroic made me work for the kill.