Countering the Protestant claim that "Oral Tradition" was invented to justify unbiblical teachings.

Protestants are understandably concerned when Catholics appeal to "Tradition" when justifying certain teachings. There is a certain objectivity, and thus safety, about having a written document like the Scriptures. Indeed, that's one of the reasons why Catholics believe God gave us the Scriptures in the first place. Too often, appeals to Tradition are framed in terms of "Catholics cannot justify this teaching from Scripture, so they must turn elsewhere," and that "elsewhere" is seen as some secrete list of teachings passed on orally, from one bishop to another, even though nobody knows when or where. If this is what "Tradition" refers to, then this should be troubling to any Catholic. But fortunately, that's not the case, and in fact the answer is deceptively simple: Oral Tradition within historical Christianity is basically synonymous with the Liturgical Life, that is how the Sacraments are celebrated, the Liturgical Calendar, etc. These are very 'public' sources to consult, and more or less objective as well, so there is no hiding things and then randomly appealing to some unwritten unverifiable "tradition" when need be. 

Why Protestants reject the Council of Nicaea.

The Council of Nicaea is a very useful apologetics tool beyond just discussing matters of the Trinity. What most people don't realize is that Nicaea produced more than just a Creed, it also issued 20 canonical laws which were binding on all Christians living at that time. The information contained in these Canons is just as useful (if not more so) as any Church Father when it comes to evaluating Protestantism against the bar of Church history. Below, I will mention why each of the 20 Canons are incompatible with all Protestantism in one way or another. This leaves Protestantism in a significant bind, because virtually all Protestants wholeheartedly affirm the Creed and deem the Council to be an orthodox testimony in early Christianity. After reading these Canons, the Protestant must recognize that they cannot embrace the Creed without also embracing the Canons, because if the Canons teach heresy and error, then the Protestant has no business at all embracing the Creed which this same Council produced. Protestants have no problem affirming the Catholic Church is correct on a lot of things, but they say the Catholic Church is false and cannot be trusted because it also teaches many errors. By this same logic, a Protestant must reject Nicaea as well, for Nicaea teaches many "errors" in its Canons and binds all Christians to these "errors" as well.