All this game does past level 60 is add more and more armour to these turtles/tortoises so that even if you're upgrading your base damage, the amount of damage you do to them steadily declines. It's pretty boring and doesn't make for a good game.
Odd that you'd say I'm doing something wrong in the same place you say this is a demo. If you can't understand something, perhaps you shouldn't make such definite statements, huh? I guess you're "normal" though, so the madness of Trump being POTUS isn't a major cause for concern. Or global warming. Or insisting on keeping murderers and other bad criminals alive with human rights when their victims get no rights. Are you okay bud?
It's as I said. After you've "finished" the game, after level 60 the tortoises gain more armour. It's like making a game steadily more difficult simply by giving enemies greater HP. Or the difficulty level in a game determining the same. It's too basic.
This by now is an old demo, too. Perhaps the dev is uninterested in working further on it. They shouldn't be surprised if they get comments that criticize it for pretending to be one thing but falling short of anything excusable by the word "demo".