I used the beginning of mission 8 on hard difficulty. I had about 10-15 deaths from playing story mode/getting killed doing the last mission and boss fights on hard. It took about 1hr and 30mins, maybe less but definitely over an hour.
Just left my Xbox on and every 10mins or so, tapped the analog sticks so the controller wouldn't disconnect and pause the death farming.
Quite enjoyed completing this game but the death achievement was a pain, hate achievements like that.