Thankfully due to the power of reddit the heroes have been drastically reduced. This is looking like a 25/30 hours investment now.
There are a total of 14 heroes. Not all are available straight away.
Unlocked: the following heroes are unlocked right away
The following need unlocking via credits
Darth Vader 15,000
Iden Versio 5,000
Emperor palpatine 10,000
Leia Organa 10,000
Luke Skywalker 15,000
Playing multiplayer, completing arcade games ( at least the first time) and challenges will get you credits. Challenges are the biggest and best source of income, focus on these to reduce time on this chievo.