I DID IT! After 11.5 months and apparently a tracked 1,926.12 hours, I got Seriously 3.0 for Gears of War 3. I'm off to introduce myself to grass now.
Complete Mission Mode playing as Ivar or Gullarp - Small Arms
Play online to 'catch' this elusive Achievement - Small Arms
Defeat 50 opponents in Challenge Mode - Small Arms
439/439235/23514/14
3,208/3,5581,200/1,25056/57
3,754/4,8151,250/1,50055/65