General Tips
Survival is the key to success in Alien versus Predator, and a player won't survive long if his army is weak. Play through all the tutorials before attempting the campaigns. These tutorials give a few strategies for beginning players and explain in detail how to use each unit of your group to its full potential. Keep your team together in a group so the enemy never outnumbers you, especially in the beginning of a mission. Try to avoid enemy encounters until your units are upgraded and you have reached the population capacity of your team.
Marine Tips
Marines are the weakest of the species in the game, so make sure you have at least two medics in all your parties. More than for other species, it is important to keep your troops together. Since most marine troop types have ranged attacks, keep your group in open spaces to spread out your enemies. Do not use the flamethrower troops unless your campaign supplies them, since they can explode and damage the other members of your party. Have at least two commtechs in your group to call for new recruits so you never run out of men.
Alien Tips
Aliens spawn more troops from host bodies with the facehugger troop. The troops spawned will vary depending on your facehugger. Make sure there are facehuggers in all your parties to quickly outnumber your foe. Defend the facehuggers until they spawn a new troop member. Upgrade your drones first, since the upgraded drone can poison your enemies. Always make sure your party carries at least one drone and one runner to ward off enemy attacks. Ravagers and Queens are your strongest units, so build your party towards these troops.
Predator Tips
Predators have the strongest troop types, but getting new recruits is expensive. Keep your troops alive by hiding from the enemy and finishing them off one by one. Since the enemy can destroy predator shrines and keep you from ordering new recruits, protect these locations for the duration of the campaign. Most predator troops are cloaked from enemy sensors, but you are revealed by attacking enemies. Do not move after killing an enemy with a spearmaster troop to make sure you are not seen.