Database
[{_id:1,x:75,y:"y",z:"z",extra:"First doc"},{_id:2,x:50,y:"y",z:"z",a:true,b:"value",extra:"Second doc"}]
Query
db.collection.aggregate([{"$match":{"x":{"$lte":100}}},{"$sort":{"x":-1}},{"$group":{"_id":{"y":"$y","z":"$z"},"r":{"$first":"$$CURRENT"}}},{"$replaceRoot":{"newRoot":"$r"}},{"$match":{"a":true,"b":"value"}}])