Took me a while to find it, but the following sequence solves "harder" without losing lives, using restarts, using clovers, selling towers, or unlocking Fire or Energy:
Sling x2 : Crossbow x2 : Acid x3 : Boulder x3 : Fountain x1 : Water x3 : Ice x12 : Stone x10
So the game probably needs an "even harder" mode :) Potentially unlocked together with the Endless mode.
Also would be cool if the final credits sequence recognized the number of remaining unlocks, and whether the towers were sold / moved during the game.
Some other fun challenges:
1. Using only one tower supertype. Earth is enough to solve "harder", but I was not able to solve it with any other kind alone (I suspect that Water is doable);
2. Using only two towers types. Acid + boulder and acid + water are both enough to solve "harder", but the last two waves require a lot of effort;
3. Not building any towers after defeating Father.