I'm new to DRG and I need some tips for hazard level 4-5

I can handle myself at difficulty 3 just fine, but when I do difficulty 4 then it seems like I'm just overwhelmed. Some missions aren't bad and I'm able to complete it the first try, but a mission like Salvage operation I'm just seemed to get a train ran on me because the swarm me so fast and they want me to stay inside a certain area. Any tips for making this easier? So far I only play Driller.