Quoxir is the first level in which you will see the Class 4 Heavy Driller, though they are quite rare. Here your greatest threat is being ambushed by IDRs, Sniper-RFs or Weevils. The keys here are usually well locked up, but they shouldn't be impossible to get.
You must drop a force field before you have access to the Blue Keycard. The yellow keycard has several triggers that must be destroyed before you can obtain it. The red key has three of them.
Look out for Earthshaker missiles. There is only one here, but you may need as many as you can find for the bosses to come.
Class 4 Heavy Driller