Beginners should aim for Fortune Armor and the golden machine gun for efficient farming. Later, transition to the Perseus or Disc 24 to maximize your Gold and EXP gains.
The only way to get free Mithril outside of daily bonuses is by killing bosses in online co-op mode. Note that drop rates reset every 24 hours.
Invest in the FREEDOM backpack as soon as possible. It negates ammo costs in PvE and provides HP regeneration, allowing you to farm bosses for massive profit without spending a dime on energy.
Some maps, like Carriage , have known glitches where you can shoot through walls or find "invisible" spots to hide from bosses.