You would be understood, but ‘bataille de boules de neige’ is the standard expression, and changing it would sound odd. Just as you could say a ‘snow battle’ or a ‘snow war’ in English, but would be unlikely to (at least as far as I have ever heard used, being from a non-snow part of the country). 

Did you mean passé composé makes it difficult ? According to the description it is not intended to be an exercise on passé simple - if passé simple is thrown in, I would agree with you that for most, it is worth recognising but not worth learning. 

As for passé composé- it is the most commonly used past tense in everyday French, so pretty much essential to come to grips with. Over time, you will have a core set of verbs you use regularly in communication, and then learning the more infrequently used ones will also get easier. 

You will never know every verb and neither will anyone else ! (As always in French, there may be an occasional and unpredictable exception).