Here's a simple way to collect orbs rapidly. Use the Mission Select option and return to the very first level. Kill all of the spawns on this level, and complete it in its entirety. If you're quick about this, you'll get an awesome bonus for quick level completion, and you'll also gain a lot of orbs after they're dropped by fallen enemies. You can easily collect 4,000-6,000 orbs per trip through the stage. Repeating this will allow you to collect a mass of orbs that you can then use.