[{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}]}]

db.collection.aggregate([{$addFields:{difficulty:{$arrayElemAt:["$skills",{$indexOfArray:["$skills.skillLevel",{$max:"$skills.skillLevel"}]}]}}},{$set:{"difficulty":"$difficulty.skillLevel"}},{$match:{difficulty:4/** desiredDifficulty == 4*/}}])