Hazar Khall's solution is accurate. There are a couple of ways to skip the grind though.
Beating the boss of the "Aspyr's featured Session" Venture under "Featured Ventures" will reward you about 500K prestige. You'll be able to Upgrade every character and have hundreds of thousands of prestige left over.
As another alternative, simply participating in long Ventures rewards you too. If the community beats a Venture that you have spawned into at least once, you get rewards. (Even when not logged in. You'll see the rewards in your inbox when logging in the next time.) I have been rewarded 5000+ prestige for some Ventures. It's a nice semi-passive way to skip the grind. "Featured Ventures" seem to have more activity and better rewards so I would focus on spawning into each of them at least once per gaming session to maximize your chance of rewards.