If you've wrapped up everything but the death achievements in this turkey and need to grind them out like i did, the fastest way is:
1. select Survival
2. Select character El Nomad (he has the lowest health, but I don't think it makes much of a differenct tbh)
3. Mash/autofire
Survival mode works way better than story mode because there are less loading screens, the enemies come to you, and in story mode you get "cancelled" before all your lives get used up. Not an issue here. I'd estimate about an hour for all 100 deaths if you start from zero for some reason.
Happy hunting!