Yes, I did max Jenkins. The trick is the Doctor's Orders quest. You could get 4 loot midgets every 3.5 to 4 minutes per run. It took many many many runs.
Other noteable challenges include: 1. Blue Sparks (75; power sliding is tough) 2. Sky Rockets in flight (2000 long distance launcher kills; infinite ammo glitch helped with this one, else it would have taken years) 3. Longshot (5000 sniper kills; I rarely used them in game and this is a huge number) 4. Crouching Tiger, Hidden Assault Rifle (3200 crouched assault rifle kills; must be glitched or something)