i boosted in stage 37, a dock level, there are 6 tunnels spawning endless waves of spiders, ants and red ants. with 2 controllers (ranger and wing diver) i used wing diver to get there, bomb away all houses and containers there with plasma launcher and just kill and collect everything.
in 1 hour you can rack up nearly 100 - 120 HP with wing diver, and the ranger gets 200 - 240 HP.
so plan a few days boosting straight.
To get inferno weapons fast and soon, especially with air raider and fencer, play the only cave level with 2 giant ants at the end, or other caves if you think you can beat it. they can be done with low HP, so farm weapons there.
Another tip on inferno is to hide under catwalk platforms or in some houses that have a roof above the entrance when playing stages with dragons in it. they will sometimes grab you through the wall there and glicth you to death so keep moving.
Its always good to keep pushing walls or against buildings.
In the last stage you start at a park, nearby at the street is a tunnel, there are a few levels in this city, so remember that this tunnel will help you alot.
Weapons that helped me alot:
- Plasma sniper called Monster-S for dropships and bigger enemies
- Plasma spreading launcher called something like M30, 30x320 damage wide spreading
- close combat laser, one of the later ones (first category)
you can literally, even with 2000 HP, fly through the arena caves and
beat all enemies unharmed, it will paralyse them. Also it doenst need that much of the energybar.
- AF99 assault rifle (get it eventually in the cave with the giant bees)
- a grenade launcher which summons fire (very helpful with the dragons)
Fencer and air raider however will need in 2 player coop at least one guy beeing ranger or wing diver to protect and take down dropships.