A guide to using the Infiltrator class at its maximum efficiency, and handing Harbinger his sorry, weirdly glowing ass.
So, you want to know why I like the Infiltrator class so much? You want to know why I picked Infiltrator in my Insanity playthrough? Well, you probably don't, honestly, but LET ME TELL YOU ANYWAYS! It's because you can do some serious damage as an Infiltrator, if you know what skills to pick.
To start off, I'm going to assume that basically anyone reading this has already played through Mass Effect 2 once. If not, feel free to read and learn, but know that the following bit of advice won't be relevant to you. Once you've played through the game once, and you start another runthrough, you'll get bonuses from completing the game previously. One of those bonuses lets you choose an extra power from your squad's "loyalty powers" (the powers unlocked after you successfully complete the individual squad member's loyalty mission). If you plan on being an infiltrator, I cannot recommend Warp Ammo enough. I'm a fan of some of the other powers, including Armor Piercing Ammo, but trust me: Warp Ammo is the way to go. You'll need it once you actually start facing the collectors.
Oh, spoilers in this paragraph. Be warned. Once you progress far enough in the game and get to the mission on the collector ship, and you get the chance to pick up an extra weapon, I cannot recommend enough that you pick up the widow. I'm a huge fan of the assault rifle, personally, but in order to survive, especially at the higher difficulty levels, you NEED the widow.
Why, you ask? Because on almost every difficulty level (it doesn't work on insanity, and I think it might work on veteran) with the Widow, and with Warp Ammo on, you can kill Harbinger with a single headshot. That's right, just ONE. This makes the later levels of the game ridiculously easier to handle. Basically the moment one of the collectors starts getting "possessed" by Harbinger, you can scope in, slow down time, put a round in oddly-shaped dome, and kill him before he even has a chance to do anything. To anyone who's played the game through once already, I don't think I need to elaborate how useful that is.
Next up, which powers do you max? Well... If you do choose Warp Ammo, I recommend upgrading that. Not necessarily maxing it out, just upgrading it. I would definitely max out the Operative tree, mostly because it increases the "slow-mo" effect when you scope in, and I would also try and max out Incinerate and Tactical Cloak. The choice you make when maxing it is really up to you, but I'd recommend the branch of Tactical Cloak which lets you heal while invisible. It's very helpful later on.
Besides that, all I can say is always keep your squad's weapons and skills in mind, always try to have a diverse squad, and never, EVER forget you have the tactical cloak. It will get you out of some really tight spots.
Beyond that, I've got nothing besides good luck, and happy hunting.