[{"floor_level":"12-1","battle_index":1,"party":[1,2,3,4]},{"floor_level":"12-1","battle_index":1,"party":[1,2,4,5]},{"floor_level":"12-1","battle_index":1,"party":[1,2,4,5]},{"floor_level":"12-2","battle_index":1,"party":[1,2,3,4]},{"floor_level":"12-2","battle_index":1,"party":[1,2,4,5]}]

db.collection.aggregate([{"$group":{"_id":{level:"$floor_level",party:"$party"},"count":{"$sum":1}}},{"$group":{"_id":"$_id.level","parties":{"$push":{party:"$_id.party",count:"$count"}}}}])