Instead of achievements, how about call it event challenges? It would be for battles not following the standard ranked challenge format. For example, us working against a giant boss monster or army, including season related boss battles such as a possessed santa claus for christmas time. Of course the other bizarre things I mentioned could also be event challenges as well as serious events for those looking to test their mettle more than just in an even, fair ranked match. Some event challenges could also be ranked for those close enough to the format, but a lot of them would be incompatible.