When you shoot your enemies in the shin they will fall to their knees and give you an opportunity to go in for the kill. This sweet spot is in the range of the shin or knee. These are the moves:

Leon- ingame= Back kick Mercenaries= Suplex
Ada- Back kick
Krauser= Kick
Hunk the 4th Survivor= Knee
Wesker= Chikyo (Axe) kick
All are at least one-hit wonders 84% of the time