My biggest problem is just surviving them. I got to the final wave, but even with all my guys at full health, I was getting taken down too quickly. I think the probelm was that I wasn't killing them fast enough, so I spent the afternoon levelling up my BBs. I got insanely lucky so far and got Serin from 1 -> 10 and Cayena from 1 -> 5, all on about 100 energy. I think if I can finish off this Cayena and get my other Serin to 10 as well, then i should steamroll End.
Another problem is my spheres. I only had 1 Sacred Jewel. But I managed to clear Middle with 550k so now I have a second Sacred Jewel, should help me stay alive a bit longer.
BTW, did you use a healer? I can't decide what's better. Felneus x2, Serin x2, Cayena x2 or Felneus x2, Serin x2, Cayena x1, Tree x1