As you progress through Raid mode, you'll level up by clearing out stages, and getting good stage completion ratings as well as bonuses such as: No damage recieved, genocide (all enemies killed), low level bonus (beating missions that are a higher lvl than you) and trinity bonus for receiving all three bonus in one stage. As you increase your level, your defense and damage dealt will increase, as well as your ability to use more powerful weapons, allowing you to tackle more difficult missions.
To help you grind out some xp/bp (and kills), check out this video for Level 2 on Trench difficulty.
you can farm exp and money on the first stage of abyss, using Jessica and just running to the last part of the stage, to the kitchen. Level 30 or 31 will give 30000 bp and 2500 exp easily. If lower your level even more o kill more enemys the exp and BP will get even higher