Remember that for your university course, the "idioms" you should be thinking about mainly are constructions with common verbs (e.g. knowing which preposition to use with a verb, knowing that a verb takes an indirect object...).
Have a look at:
- Barron's 501 French verbs, if you're more of a "visual" learner (it spells out many verb conjugations in full + lists idiomatic expressions with each verb) -- main cons with this book are the appalling pronunciation section plus for some people having 50+ regular verbs spelt out in such a large format is over-egging the omelette slightly
- Barron's French Idioms, which is a better choice than the previous one if you're fairly sound on your verb conjugations but just want to learn about the actual associated expressions
- vocab books such as Barron's Mastering French Vocabulary and Living Language 6000+ Essential French Words
Plus alongside guides such as the above, read regularly in French about things you're interested in (i.e. not just for an academic exercise but things that you actually WANT to read) with a comprehensive dictionary by your side, and:
- look out for expressions as they're used in the text
- when you look up words, notice the examples/expressions in the dictionary entry and look out for useful idioms