How to play like I do, as a Bruce Willis looking infiltrator.
The key to this strategy is to obtain the most power sniper rifle (the Widow) and use the Assassination cloak. Using an ammo power from a squad member is probably best, I would suggest Warp. On higher difficulties, you can also have a squad cryo ammo, which your allies will use to great effect. Ultimately, the choice of ammo powers is yours.
Essentially, all you need to do is take cover, cloak, and pull off a head shot for major damage. Any other shots work also, but you obviously want maximum damage for this setup, ideally taking out enemies in a single shot. Save the cloak bonus damage for tougher enemies, you will usually take out lesser enemies with a single shot anyway. You can also use your cloak to sneak up on unsuspecting groups of unengaged enemies and get a clean opening shot, so long as you can get to cover.
If you run into trouble, the cloak is your friend. Assuming it is ready to be used, hit the button and reposition yourself, or just run as fast as you can. You will get killed quickly on higher difficulties, so make your shots count. As for squad members, I would suggest Miranda, Grunt, Zaeed, and anyone that can increase your abilities. Ideally, you will have a lot of firepower at your disposal, and you just need to stay focused and take down enemies. Try for one shot, one kill, and maybe spam a little incinerate if you get caught by a group. I suggest the wide area incinerate, just for utility.