[{_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":{"$reduce":{"input":"$key2","initialValue":false,"in":{"$or":["$$value",{"$in":["$$this",["Mermaid / Fairy","Superhero"]]}]}}}}}])