I wouldn't call luck itself a skill, but the abilities to capitalise on luck and to solve issues that might arise due to lack of luck are certainly skills.
Denken (German for "thought") might be an old shrewd man, but he's still not heartless. It's one thing to kill your political enemies to further your goals, but killing a kid (they're both kids from his PoV) when there are other solutions is another matter.
Fern's poker face comes handy at those times. Raised by a Frieren that shows almost no emotion. As well as her "stick to the fundamentals" approach, like hinted in ep19 fighting her is more like fighting an old mage than a kid.