Database
[{title:"Attack",skills:[{skillId:ObjectId("65b536c74d8fca26a62e32d1"),skillLevel:1},{skillId:ObjectId("65b536c74d8fca26a62e32d2"),skillLevel:2},{skillId:ObjectId("65b536c74d8fca26a62e32d3"),skillLevel:4}]},{title:"Defend",skills:[{skillId:ObjectId("65b536c74d8fca26a62e32d4"),skillLevel:1},{skillId:ObjectId("65b536c74d8fca26a62e32d5"),skillLevel:5},{skillId:ObjectId("65b536c74d8fca26a62e32d6"),skillLevel:4}]},{title:"Chill",skills:[]}]
Query
db.collection.aggregate([{$set:{maxDifficulty:{$first:{$sortArray:{input:"$skills.skillLevel",sortBy:-1}}}}}])