It's best to do this with 2 to 4 players over XBOX Live. The general strategy is to let 1 person take all of the upgrades (for the normal gun and armor) and let the other player/s grab the other weapons. Do not let your main upgrading player die. For weapons, it's best to use either Lightning, Shield, and/or Laser (switching when needed). Once the main fully upgrading player has all of his armor (spikes) and guns (4-way shot), and doesn't need to be healed, let another person have the upgrades but also use the armor drops for healing as well.