You've decided it's time to pull out the big guns and get that scout report! First thing you need to know is you need attack heroes not intelligence heroes. To date I haven't heard of anyone losing a hero when scouting so I've concluded it's safe to send your most powerful attack heroes without worrying about losing them.
Next you need to assess how many scouts you think he has. Is it under 100k? If so your task is simple. Send more scouts than he has and your best attack hero and you'll get a scout report. If he has more then it get's a bit more complicated.
If he has between 100k and 125k you can always use a war ensign to scout him with. Personally I think this is a bit of a waist of a very powerful item. And then what would we do if he has more than 125k scouts? Well the simple answer is we need to overpower him with pure numbers. Generally you're going to kill about half as many scouts as he does of yours if he has more scouts than you. Thus for every 1 scout above about 90k scouts he has you'll need 2 scouts. Then round up to the nearest 100k.
Example:
opponent scouts: 130k
needed scouts: (130k – 90k) x 2 + 100k = 180k ? round up ? 200k
Once you have those scouts you need to scout him first with your most powerful hero then with your next powerful hero progressively in 100k scouting missions. Make your scouting missions packed together (time wise) as close as possible to stop your opponent from resurrecting his scouts and messing you up.
You should be able to get a successful scout using this method. You can even use multiple people together to pull this off. Best of luck!