Infiltrator + Kuwashii Visor + Tactical Cloak + fully upgraded sniper rifle = ass kicking one shot sniper kills.
Step One: Become an Infiltrator Esckewing traditional soldier tactics and flashy biotics, the Infiltrator class lets you feel like a true combat god, eliminating targets before they even know you're there.
Step Two: Kuwashii Visor Pick up this essential piece of equipment on Omega to increase your headshot damage. Not just useful for targeting, it is also probably the most fashionable of headgear choices, allowing your gorgeous Sheppard's expressive face to remain front and centre.
Step Three: fully upgrade Tactical Cloak Tactical Cloak is one of the best powers available in any class in ME2 (and I've played them all). Not only can you perch yourself as you like and get out of sticky situations by essentially becoming invisible, at full strength and upon choosing the Assassination Cloak option, your damage is increased +75% when cloaked.
Step Four: upgrade your sniper damage There are many upgrades you can buy through the Tech terminal that will take your Sheppard from sniping legend to sniping god. Never skip a Sniper Rifle upgrade!
Step Five: Become the Widow maker To truly effect one shot kills on even the toughest of creatures, you need an ultimate weapon. The best is the M-98 Widow, available as an option on the Collector Ship for Soldiers and Infiltrators. This huge sniper rifle slices through everything: armor, shields and biotic barriers. As it says in the description: "no ordinary human could fire it without shattering an arm", but you, Sheppard are so far beyond the ordinary.
Step Six: Enjoy. Take your weapon, equip and upgrade for damage and revel in the delicious one-shots deaths you deliver.