I've only tried one of the others and it was ridiculously hard. I forget the name but you have to escort this tank through the city along a road full of mines, which you have to shoot as you go but you also have enemies fecking everywhere on all sides, behind you, ahead of you, to the sides with RPGs trying to blow the tank up too. We got it there limping over the finish line on fire a few times, then a new phase starts where you have to defend a location from waves of enemies, which we just about managed, then you move inside to a warehouse and defend another place for wave 2 which we scraped through, then it asks you go to go location 3 - we all died and rather than any kind of checkpoint it put us back to the start escorting the tank again and we got pissed off and left it after that. That one is really hard it's just constant bullets and RPGs from all directions.