A Cut Above:
Stab a Spy who is using a Revolver.
Backstab ten different people on your friends list
Kill someone within 30 seconds of them setting you on fire
Come in From the Cold:
Backstab a nemesis. (Maybe you had to be dominated whilst not a spy, then switch to spy for vengeance?)
Backstab an Engie whilst he is still building a device.
Backstab an enemy disguised spy.
I have a feeling this has something to do with an alternate knife, since that doesn't look much like the regular knife. Could this be the item that lets you steal the identity of the person you killed?
Die Another Way:
Kill a Sniper with a Razorback without firing it off.
Revolver 50 peeps
Backstab a medic with full ubercharge. (I wonder if this will count for Kritzing medics, since they can still be stabbed?)
For Your Eyes Only:
Do The Disguise Kit Taunt in a Sniper's Death Cam photo.
FYI I am a Spy:
Get some kills without going into disguise?
High Value Target:
Dominate Three or more people in a single round.
Kill someone, then change to their class w/ Disguise Kit. (Or kill someone while you're disguised as them)
Stab someone while you're being healed by an enemy medic.
Is It Safe?:
Capture 50 points. This seems rather boring...
You and another spy deploy a sap within three seconds of each other.
May I Cut In?:
Backstab someone and their medic within thirty seconds.
On Her Majesty's Secret Surface:
Be the first person on your team to get to a capture point
Defend 15 points
Deploy 1000 saps.
This looks like this alternate knife again, my guess is sap an engineering building, then backstab the engie that owns it.
Revolver 10 snipers to death
Slash and Burn:
Very abstract picture... backstab someone while they're on fire?
Looks like alternate Knife again, maybe it does ongoing damage, and this is the achievement for killing someone after you die, like Dead Heat.
Spies Like Us:
Backstab an invisible enemy Spy. (I've done this before. Alternately, grinding going invisible. There's a test of skill.)
Get 1000 backstabs with the new knife.
The Man from P.U.N.C.T.U.R.E.:
Grinding Backstabs, maybe 5000?
The Man with the Broken Guns:
Backstab an engie, then sap three of his buildings.
The Melbourne Supremacy:
Dominate a sniper with Backstabs. (Or, get revenge on a sniper dominating you)
Stab someone three times? (Three Spies facestab the same guy?)
Killing someone outside in the rain in that arena map? The name says underwater, but the picture says rain.
Who’s Your Daddy?:
Backstab a scout with his bat.
You Only Shiv Thrice:
Backstab three people in ten seconds.