Use an incendiary cannon, two barrel shotgun, sledge hammer, and 35 stimpacks. This was done with a level 9 Cain under the normal difficulty setting.
Easy Experience
In Chapter 4 in the garden, you have to escort a technician through a vault to open a escape hatch. Before you do this, make sure you have a Tesla Cleaver and the quest from the merchant to collect computer parts. Let the technician die anywhere in the level, then go to the last room (a room with three computer consoles and three robots that respawn ). Kill one of the robots and follow it back to see which laser barricade it came from. Stand about one character length from the lasers. The robot will respawn. Kill it as soon as it does. They are worth 50 experience points and sometimes drop computer parts. Do this as long as desired. When you are done, just go back to the vault and trade in all of your computer parts for 100 experience and about 30 caps each.
Unlockable Characters
To unlock characters, complete the criteria listed:
Patty: Beat the first chapter.
Rhombus: Beat the second chapter.
Vault Dweller: Beat the game.