This is a clear and semi-in depth way on how to play Infiltrator.
First things first, we need to understand the Abilities and Weapons available to this class.
Weapons: Pistols, Snipers
Abilities: Disruptor Ammo, Cryo Ammo, Tactical Cloak, Incinerate, and AI Hacking.
As an Infiltrator, your main goal is to single out units with tactical use of your abilities to defuse their chance at attacking/counter-attacking effectively.
The Sniper is your main weapon of choice. Mixed with various abilities that allow for surprisingly effective damage, the sniper is the perfect compliment.
-Recommended Choice: M-98 Widow-
This weapon has the highest damage output among all available snipers. With the use of Tactical Cloak, dealing massive one-shot kills can be accomplished much easier.
The pistol is your back up weapon when sniper ammo is low or out.
-Recommended Choice: M-5 Phalanx-
This pistol also has the highest damage output along with a moderate ammo size.
Side Note: Though it may seem these two weapons combined do not carry enough ammunition, your Tactical Cloak and Incinerate abilities combined with your allies abilities should be enough to help you manage your ammunition needs.
Assuming the enemy force is loaded with shields, it is best to activate distruptor ammunition to tear through them.
When the enemy force is loaded with armor such as Krogan and Vorcha (who also have regenerative abilities), it is best to use incinerate. Not only does Incinerate destroy armor, but it also decreases the rate of regeneration in enemies.
Cryo ammunition is best used when the base health of enemies is shown. It can narrow down the amount of attacking forces on your position so you can get a clear shot opportunity to take on other forces and thin them out.
AI Hacking is a great tool for thinning out hoards of robotic enemy forces. Reducing the mechanical unit to its base health allows you to take momentary control of that unit and add it to your force. Not only will that unit attack nearby enemies, but will also take on fire, making it easier for you to take it out when you lose control of it.
The Tactical Cloak is the bread and butter of the Infiltrators main Arsenal of abilities. For a short while, you will be literally invisible to enemy forces, allowing you clean shots at enemies without rebuttal. After use of another ability or shot, the cloak will prematurely deactivate. Combined with the sniper rifle, you will be able to line up headshots for critical damage.
Side Note: When the amount of forces recedes to just a few and they are spread apart, it is sometimes viable to use your Tactical Cloak to close the gap between you and one of the open enemies and take them out from close range in order to conserve rounds for your sniper.
Because the infiltrator class is a mix between tech and weapon usage, it is recommended to have a blend of biotic using teammates in your squad to balance everything out.
Recommended squad mates include: Miranda, Thane, Samara, Jack, Jacob.
Side note: As an additional ability to use for Shepherd after gaining loyalties from squad mates, it is recommended to take either Warp Ammo, Slam, and Dominate. These 3 abilities further compliment the Infiltrators abilities to quickly disrupt the enemies chance of counter attack.
Warp Ammo: Crushes Barriers that numerous enemies will have. Especially recommended near the end of the game where almost every enemy will have a barrier.
Slam: Used to lift and slam enemies to the ground, effectively narrowing down the amount of enemies firing at you.
Dominate: Used much like AI Hacking, but for Organic units. Use this to place focus on a different unit so you can line up headshots.
I hope this guide was useful in teaching you to effectively play the Infiltrator Class. Enjoy!