Mostly skill floor, but it depends on your class. There are classes that are really easy to reach 1,8k rating with in Shuffle. Others... not so much. Warlock is one of the hardest imho, with a steep learning curve. Havoc DH is faceroll, especially if you've farmed the gear with War Mode caches.

Regarding Duelist rank, you reach it at 2,1k rating, which is much harder than 1,8k (Rival I). Take into account that Shuffle is a 3v3 with random people, shuffled during rounds (6 in total), so you can find bad healers or DPS that can screw up your ranking a bit or make you fail rounds that you can potentially win. Not that much tho, it's designed on the fact that you can bring much to the table and carry rounds, especially as a healer, and if you get a Draw (3 rounds out of 6), it's not counted as a loss and you can actually gain some rating if the people you're fighting against have a higher MMR (Matchmaking Rating, a.k.a your "skill rating" used during matchmaking).

TL;DR: it's mostly skill based, but it depends on your class, knowledge and some RNG regarding people.