Originally posted by The Baron
Fuel rod gun is also, IMO, highly overrated. Pistol and shotgun are a fantastic combination; use the zoom on the pistol to strip their shields, and then when they get near you, whip out the shotgun and kill them in one shot.
If you're good enough, you can kill them in one shot with the shotgun without stripping their shield, but it has to be point-blank (Something which is difficult to perform when playing in a non-LAN environment).
Good tip for online play, however.
Quickest death possible with each weapon is:
Plasma Grenade : (Tag) Instant death. Even when opponent has a full overshield.
Frag Grenade : 2 consecutive grenades at any range. First will leave your opponent with 3 health / no shield (If it's a direct hit). Second will kill.
Melee : (From Behind) Instant death. Even when opponent has a full overshield.
Pistol : 1 head + 2 body.
Shotgun : 1 body shot point blank.
Fuel Rod : 2 shots (any range).
Rocket : 1 shot (any range).
Needler : 3/4 Clip (any range, all needles must make contact however).
Plasma Pistol : 1 fully charged shot + full cell. You can use a fully charged shot + melee for same effect. (More reliable)
Plasma Rifle : Takes approximately a full Cell at point blank (This is up for debate, however).
Assault Rifle : 3/4 Clip at point blank.
Sniper Rifle : 1 head shot at any range.
Other than the Plasma Pistol/Rifle which are educated guesses based on the time (3+ months) I've spent with the game, these are accurate "quickest death" scenarios.
Hope this helps those that didn't know before hand.