This massive horde will attack almost immediately after sending a distress signal. It is composed almost entirely of Agrons and Infested Marines. Tall walls are required, Agrons will give sight for their ranged allies otherwise. The Stinger will grind the Agrons into dust, but Infested Rines will shoot some of the rockets down. This horde is very easily dealt with, and serves more as a last chance to reach T3 skills before Nazara than anything. Make sure that the character that needs Experience the most receives the Stinger. No one wants to be underleveled.
Once the huge horde has been properly dealt with do not lower your guard. Several Tartarus will emerge from Apollo and move towards the team. They are easily dispatched by high level skills and the Stinger, but they could kill a lone player if he was unobservant. Once the beasts have been dealt with it is safe to proceed to the Laboratory.