Multiverse Madness mode is similar to Firefight mode in Halo or Horde mode in Gears of War. What worked for me was to play on the Space Station map and stick to the starting area (in the room near the Joe special item with the health pack special item in it). this way I could force the enemy waves to funnel themselves to me through the room's only entrance.
I played as Quagmire and once I had enough cash I purchased the acid sprayer and watched my enemies melt away. But you can also do this with Stewie and use the Magma gun.
I also purchased landmines and placed them between waves to cause more destruction.
For a faster reload, you can stand by the store portal and then enter the store and cancel out.
After 15 waves, this will pop. If you continue on through waave 30, you will get: