If you're fighting on Ludicrous, you should face slightly weaker heroes than you in the first two waves, equal hero strength in the 3rd wave, and stronger ones in waves 4 and 5. If stronger lineups don't exist, then you should just get equal strength lineups from waves 3 to 5. What happens now is that you face weaker ones in waves 1 & 2, equal ones in wave 3, and stupidly weak lineups in lines 4 & 5, which is the result of the system circling back to the beginning past wave 3. It shouldn't do that. Besides, there are probably a gazillion good players with strong lineups that could provide enough of a challenge for nine equally matched lineups.
In all honesty, I sometimes see lineups that never get harder than Orange+1 no matter what wave I'm in. Other times, the worst lines are in the 1st wave, and then it's easy-peasy from wave 2 onwards. I've got around 15 Cyan heroes and can handle pretty much anything thrown at me once I have everyone's energy topped off. I end up just using Medusa and one blast from Moon Drake's active skill and everyone turnes to stone and dies - repeated 15 times. The other characters might get a sucker punch in here and there, but they're mostly along for the ride. I'm not complaining about the ability to always get max prizes, but it would be nice to see it slightly more challenging.