[{_id:123,key2:["Mermaid / Fairy","Superhero"]},{_id:456,key2:["Slug"]},{_id:567,key2:["Slug","Grimace"]},{_id:678,key2:["Slug","Superhero"]}]

db.collection.aggregate([{"$match":{"$expr":{"$gt":[{"$size":{"$setIntersection":["$key2",["Mermaid / Fairy","Superhero"]]}},0]}}}])