How to get 10 lifes and infinite ammo in singleplayer:

1. Start game in multiplayer with 10 lifes and infinite ammo.
2. Quit from multiplayer.
3. Start any level in single.
4. Congratulate! You have 10 lifes and infinite ammo in singleplayer.

NOTE! Infinite ammo in singleplayer can be reached only in Steam version of SS2.

Early unlock of any level:

Finish previous level in multiplayer and return to singleplayer. Required level is open!

Example in 2:49:

I finished "Jump in the Fire" level in multiplayer. Then the next level "Siriusopolis Downtown" is open in singleplayer.