S.T.A.L.K.E.R.: Shadow of Chornobyl

Can be done very quickly but you need to advance to the point in the story where you exit X-16 and have to return to the Barkeep. On your way back in the Wild Territory, hug the left where you originally found Kruglov (escort to Yantar mission) and you will be rushed by rodents for the first time and there will be 10-20 of along with possibly other mutants as well. I gathered them up, hid in a corner and saved my game. Killed them all, got 8% each time, then reloaded my save. The rodents are small so doesnt take much ammo either. Cheers

Cumulative across gameplay, so you can reload your saves to get it.

For me, a lot of mutants spawned near the Barrier as I returned from the Red Forest. Otherwise, there should always be a pack of blind dogs and a pseudo dog near the Bar which yields around 2-3% for killing them all.