[{_id:ObjectId("633199db009be219a43ae426"),name:"Max",examScores:[{difficulty:4,score:57.9},{difficulty:6,score:62.1},{difficulty:3,score:88.5}]},{_id:ObjectId("633199db009be219a43ae427"),name:"Manu",examScores:[{difficulty:7,score:52.1},{difficulty:2,score:74.3},{difficulty:5,score:53.1}]}]

db.collection.aggregate([{$unwind:"$examScores"},{$group:{_id:{name:"$name"},maxScore:{$max:"$examScores.score"},data:{$push:"$$ROOT"}}},{$project:{_id:0,name:"$_id.name",maxScore:1,difficulty:{$getField:{field:"difficulty",input:{$getField:{field:"examScores",input:{$first:{$filter:{input:"$data",cond:{$eq:["$$this.examScores.score","$maxScore"]}}}}}}}}}}])