I finally made it to level 25 in German!
Seeing your level, I'd assume you already do plenty of review on top of the lessons, which is very good. While I was doing the German tree, the main things I did outside Duolingo were Deutsche Welle (http://www.dw.com/en/learn-german/s-2469) and Memrise (https://www.memrise.com/). Memrise was to memorize the Duolingo vocabulary and Deutsche Welle simply has very good podcasts that let me get used to the spoken language.
A lot of things in German can be very confusing at first, it took me a long time to get a hang of this "hiermit", "dazu" concept. To help you, you can check out Deutsch Für Euch (https://www.youtube.com/playlist?list=PLDl7JofqmDnHxxS7NjiGgEapMeTQyPK5R) and pick out the grammar episodes, which make grammar a bit more understandable and fun. If you need to review a type of grammar, see here: http://www.dartmouth.edu/~german/Grammatik/Grammatik.html
Otherwise, if you're trying to get used to these difficult concepts like declension, word order, etc. a great way to do it is through reading. You can start by searching online for some reading material, if you need basic things you can always start with children's stories (maybe this?: http://www.ukgermanconnection.org/?location_id=922). On Deutsche Welle (in the B1) section, there is slowly spoken news with simpler language that you could make use of, but keep in mind that the level here is pretty high. Also, as far as I'm aware, the Deutsche Welle podcasts all come with a written script that you can read along with and gather knowledge more directly. If you're having trouble with adjective declension, just look the charts up, it clarifies so much.
This post turned out far longer than intended... Oops!
Hey, I would recommend to try "Michel Thomas": http://www.michelthomas.com/learn-german.php It's the most efficient way by faaaaaaaaaaar to get going in any language, including German. It's like a virtual class; you'll master most of the grammar without any memorization and pain at all.
Another source of learning that I'd definitely recommend is "Assimil German with Ease"; it's also a good resource. It contains 100 conversation with practices and answers, as well as grammar explanation. This method, like Michel Thomas doesn't require any memorization, only the power of immersion. :-)