Really? Solo its easy.
4 man, i've only seen other do i and it looks easy enough, if you dont run around like a muppet and get yourself surrounded. The hardest part seems to be the multiple sledge attack, but i've yet to see anyone deploy flame turrets properly or use grenades and special weapons, or the choke point.
the hardest part is at the very end 2 full squads of mostly elites enter in the center parts of both sides of the map and spread out quickly. Most of the time splitting you into 4 individuals instead of a group. I been in groups that handle that transitions beautifully but most groups just get wrecked at the very last stage.
There are two sections of the mission that can go terribly badly if you don't have a plan and everyone just does there own thing. The gates where three heavies rush you is one, and the final area the other.
At the gates you need to exploit the choke point and not push into the map. You focus all your fire into that small area and if a heavy makes it in you use special ammo as a team to quickly take them out. And it has to be quick otherwise reds and purples will flood into the area and it's chaos from there on in. Flame turrets, chem launchers, genades and anything else you have that puts out a lot of damage dumped into that small space will keep the waves contained.
The final map is tricky if you trigger too many events at once or try to push into the centre of the map. It's vital to stay as a tight four and watch your corners. Again heavies are a priority to take out quickly and you need to keep an eye on the boat because a machine gunner in there and heavies trying to crush you is a nightmare, and it's worse if they put robodogs onto the field too. You need to quickly react as a four when a new waves comes and move to the opposite side if possible, but when they come from both sides you rush back to the start point.
Teams wipe in there because they get split up once overrun and don't work as a unit. A good team can do that map comfortably but a bad one will suffer wipe after wipe.