My advice is quite different. On harder difficulties it’s unlikely you will be able to hold off the enemy on two fronts with the weapons you have. You won’t have enough ammo and the enemies will probably overrun you. Better to pick one side early on and make an aggressive push, with one team mate guarding the rear. By the way I’m specifically referring to master difficulty, in case anyone is wondering.
Class/character combinations can vary, but Keegan/Tactician is excellent for this. There are no supply rooms and aside from the initial crossroad area, there are no weapons to pick up aside from what you can lift off dead enemies. Keegan can punch holes through enemy encounters using the Salvo and you will need to bring this along with you by lifting/dropping it over the wall to pick up from the other side, but needs the team to time their pushes so that Keegan can make the most of his Venom Boost card. The idea is to try and drop the Ammo Resupply in a combat situation so you can replenish Salvo rockets as you fire them - effectively giving you infinite rockets for the duration of the ammo drop. If you’re ahead of the venom, don’t be afraid for one person to trigger the next enemy encounter and lure them back a bit. Watch the venom and think about timing this so you’re not too deep in the venom, and your surroundings so you can use the environment to create kill zones.
Cole/Brawler is good too, but players need to be more patient and sensible. The enemy stopping power will have a big effect.
Lahni/Blademaster is also very good - not as tanky, but fast hit and run strikes can be very good for bleeding enemies out.
Clayton/Gunner is also very good. A combination of Heavy Charger and Ultimate Battery can charge everyone’s ultimate abilities; and his ultimate coupled with Reflect Shredder can help clear lots of enemies; plus it has a fast recharge.
I’ve seen Mac/Anchor used as well and as long as someone is watching their back for Juvies, they can unblock your path if there;s too many Drones pining you back. His ultimate is very good and also recharges fairly quickly.
My A Team suggestion would be Keegan. Clayton and Lahni - that’s my preference, but as I said, other combinations work well too.
For the 1st section, there are two methods which work well.
The first method, is to wait for the venom so that the Hivebuster characters (if any are being used) can make the most of it and fight your way through. Keegan can normally punch through the enemies ahead but fire Salvos sparingly at first as ammo is limited and his ultimate won’t be ready straight off. Lahni can guard the rear as you press up.
The second method, is that you can rush without the venom. It’s a bit riskier on the surface as the likes of Lahni won’t get the benefit of her venom cards, but the Mulcher Scion (plus some of the other enemies) that spawns as you approach the saferoom spawns on a timer, so if you’re quick you can pass that section without fighting as many enemies. This method relies alot on Keegan to conserve and space out his Salvo shots as he will be relied on for the main push, and Clayton using the Trishot to help the push too. Lahni needs to time her melee attacks on Juvies and to not overcommit to attacks as you’re not in venom so she won’t have as much defence, won’t be able to bleed enemies, and will be relying on others to cause the enemy to bleed (which will trigger her Thrill Of The Hunt card).
The 2nd section is mostly an exercise in Keegan using the Salvo while trying to stay in venom as much as possible to recharge his ultimate. Lahni mops up; and Clayton at a push can use his ultimate to rush the enemy and deal with crowds. Lahni should try to safe her ultimate for the last encounter before the helipad - you may have to make a last hard push if the venom is creeping up behind you and her Shock Chain is excellent for crowd-control. Just remember that the secondary lightning also causes bleed damage if in venom.
I think my main advice, is to not underestimate the stopping power mutator. The moment you take your first hit, you need to think about getting into cover. Time your runs and flanks for when there is a break in gunfire. If there’s too many Drones firing, then shock grenades or flashbangs can be great for creating openings. Grenadiers will push you aggressive;y, but if you can isolate the Elite Grenadiers and meatshield one (which in itself can be potentially risky as they tank melee attacks) you can nab a flashbang to use.