Oh sorry, I suppose it's not really clear.
You have to put a lot of cacti in front of the dinos. The more they trip, the more energy they lose. And they lose more energy the bigger the cacti.
You can note that they lose energy because of the mini-cacti they have attached to their bodies.
I suppose the tutorial wasn't nearly clear enough. The tutorial says that you have you have to put "lines of defense", i.e. columns of cacti, but to be able to stop a dinosaur you have to put a row of cacti, i.e. many cacti in front of him. So that was a mistake on my part.
Thanks for feedback. Appreciated!