Is this possible to master skills in another way than translating sentences?
I usually go through the lessons and test out. I haven't done much translating. My main desire is to improve my listening skills, and translating doesn't help me much.
In this case I suggest you to listen to podcasts ( for example: http://spanishpodcast.org/), because duolingo's computer voice is fine and handy to obtain a first impression, but nothing compared to native speakers. You can listen to episodes while you read the pdf's, so that the learning effect maximises. It's hard, but definitly a good investment when you learn along with duolingo...
Well yes you can keep plugging through the skill tree units. I didn't even realize I was supposed to be translating sentences until I had "mastered" quite a few units. I think I have figured out that I move faster though when I spend time translating. You get points counting towards the skill tree at the same time as you translate.
I just did a little exploring on the black bar at the top of the page. The direct links to Translations and Vocabulary give you some additional ways to improve.
I agree with rspreng. I've tried to do the translations, but you never really know if they are correct. There are tons of Spanish podcasts out there on the web. A lot of them are not for beginners, because of the extensive vocab, but it does train your ear even if you don't know everything they are saying.