Only "Body" left before Gen. Plural
God help me. Gen. Plural is labeled with a skull image, how appropriate.
The word "мечта" (dream) does not have Gen. Plural at all ))) When I was young and went to schooI I asked my Russian teacher what is the Gen. Plural of this word. She said that this form does not exist and I should say "мечтаний" instead (but "мечтанье" is a separate word) )))
У рыб нет зуб... у рыбов нет зубов... у рыбьев нет зубьев... У РЫБЫ ЕСТЬ ЗУБЫ!!
(The proper Russian for "Fish don't have teeth" is "У рыб нет зубов". But of course they do have teeth :-))
Good luck with Genitive Plural! I guess the best way to learn that is to make a table of all the nouns you know by now, and look for their Genitive Plural forms. You'll find some patterns, so it won't be memorizing totally different stuff for every noun. We Russians do the same with the English irregular verbs, by the way :-) Make tables and drill them.
Seriously, though, don't panic, just read the explanations, do the exercises and give yourself a break if not everything clicks first time round. You will be okay, it isn't impossible!
Ps I always kind of liked gen. plural - I know, I'm weird. I just like things that are a bit different. I like especially how for feminine plurals you lose a sound for the plural, which blew my brain (in a good way) the first time I learned it!
"A Short Russian Reference Grammar" by I.M. Pulkina. Here's a picture of the centerfold: http://s29.postimg.org/8i67v3odz/photo_5.jpg. Looking through it again, this is an awesome book! There is a great table that shows prepositions and all the cases they govern with examples, e.g., "за", both accusative and instrumental, answering куда (accusative) vs. где (instrumental), purpose (instrumental, following verbs of motion).