Cards used:
Towers: Bolas Tower (Level 2), Lightning Tower (Level 3), Slowdown Tower (Level 4)
Heroes: Centurion (Level 2), Sapphira (Level 2), Umbra (Level 1), Valorius (Level 1)
Avatar was Level 13. Several Hero revives were necessary.
See image below for layout at least by Wave 5. Lightning Tower was eventually placed in the vacant Tower slot on the far right.