if there were a longer mode where achievements gave you upgrades, such that it was extremely extremely difficult to win without any achievements, and such that it was long enough to get really powerful, that would be really fun. even if the sprites were repetitive
for the hard badge just build a fluffy base with lots of empty spaces for the first 250 units and then stack tall pieces in a single tower for the rest. do it carefully. i get 500-650 height most of the time this way, with plenty >600