The secret levels can be unlocked by getting an S rank in all previous missions. Mobius can only be unlocked by collecting all sixty gold rings and all four secret colored rings. It consists of four boss fights, so each fight contains a cache of ammo.
Note that if the player does not run for the exit as soon as all four bosses are defeated, they will become trapped in the level and killed.
Average Payout: 0 credits
Boss Strategy: Shadow
Shadow teleports around and primarily uses a fireball spread, a dark plasma orb, and a ballistic attack similar to a sniper rifle. If the player gets too close, he will use a strong melee punch. When his health gets critical, he will use a flame wave attack that does massive damage if the player does not dodge or jump over it. Fortunately there is a lot of cover, and Shadow has a hard time tracking invisible opponents. Summon the Heaven Guard and attack him from long range. Use Clock Freeze to get in a few close range hits whenever it recharges. Due to his speed, it is unlikely he will be stunlocked by a Gas Cloud or Dragonfire, although Dark Vortex can pin him to the spot.
Boss Strategy: Blaze
Blaze will spin around and spread flames in every direction. With no cover, they can be tough to avoid, unless the player deploys sandbags. Again, summon the Heaven Guard to distract her and use long range attacks. The Clock Freeze + Gas Grenade + Dark Vortex combo works on her, and Dragonfire can stunlock her if she is pushed against the wall.
Boss Strategy: Silver
Silver's primary attack is to throw furniture at the player, which can cause a lot of damage if the player gets cornered. Silver will also stun the player if he has direct line of sight. There are two alleys that can provide good cover, as it is unlikely Silver can keep both in his line of sight at the same time. Invisibility is a must. Use attacks that can keep him pinned down, like Dark Vortex and Electric Destruction, and hit him around corners whenever possible with Plasma Persecution. Let your Heaven Guard do most of the direct fighting.
Boss Strategy: Sonic
Sonic is actually the slowest of the four bosses and has no ranged attacks. Summon the Heaven Guard and Sonic will constantly attack it and do minimal damage. No particular special strategy is required to defeat him.