A good powerset/skillset for a chest-deep spellcasting assassin is the following;
Powers
[-3] Evocation - spirit power specialization +1, offensive spirit power focus +2
[-2] Refinement - spirit control specialization +2, offensive spirit control focus +2
[-2] Lawbreaker 1st - +2 bonus to all rolls for lethal magic
Skills
+5: Conviction, Discipline
+4: Athletics, Stealth
+3: Alertness, Endurance
+2: Resources, Lore
+1: Craftmanship, Deceit
Rotes and often-used spells
[rote] Shadow Stalker: a double spirit maneuver (8 shifts of power) to apply the sticly aspects "universal chameleon" and "utterly silent" to self.
[rote] Assassinate: weapon 15 spirit attack (requires 4th mental and physical stress box plus 2 milds) to telekinetically explode a head.
Eldritch Missile: Hurls tiny projectile at supersonic speed. Weapon 8.
Force Crush: Telekinetically crushes target as if by extreme pressure or gravity. Weapon 8 vs Endurance.
TK throw: Might 9 telekinesis effect. Use power 9 for what you can throw/push, control for targeting. Effects depend on the projectile rather than the spell.
Catastrophic Failure: 9 shifts of hexing, actually targeted as a full spell.
Equipment
high-quality scope
military vest; armor 3 vs piercing and blunt, armor 1 vs the rest.
silver, iron, wood and bronze slugs
thirty-pound sachel of explosive; weapon 8 zonewide
Submerged Improvement
Add Ritual Entropomancy, and raise his Lore to Good, and his spirit offense foci by +1 power and +1 control.
This guy can kill people really, really, really well. He works as follows;
0) Rule 0 is that he never misses.
Ever. His control (and thus attack) for lethal spells is +11. Against unaware targets, it is a moot point. In unprepared combat he uses Force Crush to target Endurance and thus gets 6 points more attack than
anybody has defense. Theoretically, there is still 1 in 1312 chance for him to miss if he rolls a -4 and the enemy rolls a +3 or a +4 but that's why you got Fate points for rerolls.
1) Against the average mark, he can set an ambush unobserved with his Stealth of Great and the Shadow Stalker rote. He spends a few moments getting "Focused" (discipline) and "Calling Up Power" (conviction) and then fires his "Assassination" rote at Weapon 15, Attack 15 against a defenseless target. Just about any human has their heads exploded outright. Even armor of 3 and a defense roll of +4 and a couple fate points (or a magical block item of 8 ) will force a PC to take a mild, moderate, severe and extreme consequence to survive. If he spends his own fate points here, or takes a few more moments to make more aspects, they are dead no matter what.
2) If there is the White Council about, he will use his Eldritch Missile spell to hurl bullets and thus make his kill look like a mortal assassination rather than black magic. This method is far less effective than hitting the target with the Assassination rote (weapon 8 rather than weapon 15) but on the other hand, a hurled projectile will ignore magical immunity (if you want to kill ogres), iron works extremely well vs Fae, silver vs shapeshifters, wood is good vs vampires and bronze vs greek and sumerian critters.
3) He can make assassinations look like accidents. His "TK Throw" can hurl a person off a building as if they'd jumped, throw a car off a bridge or into another car or force a car to run over a pedestrian or even make a wall or ceiling or other big object collapse on somebody.
4) If the target is on a plane or other vehicle, the "Catastrophic Failure" rote can make the plane crash or the car's steering break and send it off the road. 9 shifts of hexing could even make an old steam-powered train engine or small ship explode.
5) His Submerged version has +1 power and +1 control to his spells and access to entropomancy rituals. Not that he needs them - just for the sake of completeness.