The aliens have a programmed response to fight to the death. They will not escape the map boundary or take off in an intact craft while X-COM ground forces are in the area.
To the discerning commander, this means time is on your side. Unless soldiers are in need of urgent medical attention, there is no need to end the mission quickly.
Time should thus be used generously. Every movement will ideally end with a soldier next to cover, kneeling, and with a pause left for reaction fire. Each new position should be scouted and covered by overlapping fields of fire during the scouting. When in doubt, hunker down, take defensive positions, and wait for the enemy to move. The order to save time for reaction fire, generally snap shots, should be given at the start of every movement phase and only rescinded for soldiers in safe rear positions or HWP's that can bear the brunt of a surprise attack.
When an alien is spotted, have other scouts check your flanks, then the support team should concentrate on bringing the alien down immediately. The high potency of alien weapons means that if the alien gets a chance to return fire, especially aimed fire, somebody will die. Scouts should not try to flush additional aliens if the support team is busy and does not have the time to bring them down. Instead they should take defensive positions and wait.
The time taken to prepare variable-delay grenades can also be optimized using the "Grenade Relay" technique.