Basically anything Vs. Necrons. Doesn't matter where you are (though a WH40K-related board/shop/etc. helps with this).

Not that pitting them against various Sci-Fi forces is necessarily a bad idea, but it always devolves into "C'Tan beat anything and everything ever" even if the initial premise involves "leave the C'Tan out of this."

Sure, when it's Chaos vs. something nobody seriously contests that Khorne is going to get all up in their business, and when it's Imperium vs. something nobody brings up that maybe the God-Emperor will suddenly wake up and lay waste to everything. Why's the Nightbringer get to settle every skirmish?