Despair and Under Siege are online-only missions of EDF 5. Both are nasty, although not impossible. The main problem is that there needs to be some coordination on both. If everyone is just shooting around you'll never solve it.
The best way to tackle missions online is by each player using a different class. This way you're optimizing for a positive outcome, as each class has different capabilities, and most missions will be easier to handle if you explore them. These 2 are missions that if players insist on avoiding or concentrating on specific classes you're aiming for a huge difficulty spike and likely failure.