What do you guys mean by "no losses"?

[DGDobrev]

My thougths exactly. In addition to that, a no-loss challenge forces you to think about every single simple move you make - not playing a battle like you didn't care. You get used to your troops, you babysit them, you work at them. This helps you to figure out to the best of your ability their strengths, weaknesses and how to use them in the end for better results.

For example, I sorely underestimated the Tirexes. Now I can't do a battle without them. They are the ultimate tank against opponents that inflict physical damage. I knew the paladins were good, but these take the cake. I never lose even 1 tirex even against invincible opponents that spam geyser every turn.