I just want to say that its possible and I accomplished it today with $0 spend. My post will debunk some of the myths out there, stating you need a full roster of max out characters. And of course you don't need to spend a fortune unlocking so many characters and wasting all your credits. All you need is 10 solid characters and mix them around. I learned that by reading the forum from vets and experience players and with that knolwedge , I put them to good use.
To make my story short, it all happened when I stopped leveling my hero because I hate it that everytime I level up, I had to feed my minions training droid and coins.
I only got 2 heroes that are gear 7 and they're Dooku and JC. The rest are gear 5 and below (none purple) just for he challenge and I face level 70+ characters at node 2, and the last few node but I managed to beat them all with my all level 53 team.