Whilst I can't help you directly, one thing that's worth doing is making sure you track down any auxiliary buildings in the levels. Some of these, like the power relay, can reduce the enemy power level, making the enemies weaker. You can find them more easily by hacking radar relays.
Secondaries like EMP and Virus are very good at getting you out of a tight situation.
In co-op games, invest heavily in healing so you can get those clutch revives in.
Shops that are further from the 'main route' to the volcano tend to have better things in them.
Sometimes it's worth retreating from a particularly tough level (the later convoy levels are particularly tricky) rather than burning all your lives trying to beat it. You don't have to complete the hex if there's another way through.