Sometimes its better to think first of your actions before you pull the trigger.
The best battles are sometimes won with a strong mind. If you can handle being able to think during a battle and not fall into traps of enemies trying to flush you from your cover.
One of the best aspects of the infiltrator class is the ability to disarm your enemy's and quickly sap their health especially in heated battles. The infiltrators greatest assets are his/her weapon set up and tech powers.
These are the powers i used to beat the game on insanity and did not change throughout the entire game
Incinerate: is your friend. Its heavy impact damage and slow drainage of the enemy's health is a great asset in later battles.
Tactical cloak: is what you need to get through the game on insanity. It helps you in a tight spot if a enemy's about to deliver the final blow allowing you time to move and reset up in better cover and thus giving you more time to heal.
Reave: though not powerful it does give you a few moments of rest as it causes the enemy's to stun and tears through barriers.
Also the collector rifle, the widow sniper and any version of the pistol standard upgrade will do you great throughout the play-through. The widow is best used while in cloak as you get a massive damage boost. mix that with head shots and you will win many battles quickly. The beam rifles is better saved for once you start to battle the collectors or husk as it tears through them better than other enemy types.
Reave+Concussion shot using garrus does a good amount of damage to enemy s without shields Warp+Incinerate= noting better than floating burning enemies
Concussion shot+ incinerate= great damage output plus knockdown
As I said this class is more about sitting in cover for a few moments to learn your enemy's moves and makes plans as you take up cover and wear them down with your tech.