Honestly, for your average mortal, I just have the assassin roll an opposed stealth roll, and call the mortal dead.

For a goon that might actually take a consequence (meaning 3-4 stress, and a -2 consequence, so 5-6 stress to take out), now we'll do two rolls: sneak (as a maneuver) and an actual attack roll. With a 4-5 skill and one maneuver (so 6-7 plus roll) versus someone starting at 0, the enemy is usually taken out in a hit,  but a careful person probably wants one more maneuver there (aiming or such).

For anything that's going to take more than one consequence, now the player needs to get really creative with taking them out.  We had a sniper hit for 28 stress before without magic, but it was a series of steps done to hunt a specific target plus a 5 fate point dump... so, still, he pulled off 18 stress without fate points.  That would take out probably anything but a major NPC or a PC.

The thing to remember is that as a GM, the game goes faster if you actually DO tell your players what it will take to kill the guy (as in, this dude's just an accountant, you can kill him with a successful stealth roll), vs "letting them figure it out".  As the "mystery" will make sure that they always try for 20 some hit stress on every random accountant.

One of my players summoned a flying fire spirit and sent it after an escaped bad guy, who was on a boat miles away.  People in the area think a stray meteorite actually hit the boat, making it explode.

Killing is SO much more acceptable when you're evil.  Of course, now the white counsel has a hitman... er, warden... in the area.

Also, consider if you want it as an NPC, or how central this familiar will be.  We have a shaman in our group, and I drew up his familiar as an NPC just to feed hooks and info to the group, because I wanted some of the plot to be more obvious.

We have a lot of characters in our group, so I didn't want another thing I had to track in combat as the GM. So it just doesn't enter combat, really.  The spirit possesses any nearby dog when it shows up, so it only costs a fate point for the player to have the spirit show up if he needs it in a hurry, somewhere that a dog is unlikely to be.  (Otherwise, it's a simple ritual to summon the spirit if time is not an issue).

However, our current story arc hinges on an outsider that has been bound away eons ago by native american magic users.  Turns out, in a different time line, native american culture won out: they were more numerous and developed when the Europeans arrived, they went to war, won, conquered the world in time, and went forward past our modern day in a society based on open use of magic, led by shamans.  However, the tribes never reached true peace, and one of them eventually called in an outsider that moves through time so that his tribe could eliminate other tribes before they ever started...and it all went to pot, some shamans went into the past to imprison the thing, ending up altering the time-line so drastically that our time-line happened instead.

Anyways, so the familiar spirit happens to know all of this because he was brought back from the alternate future when the shamans went to our own past... see this gets confusing, stupid time travel... and I use him frequently as a way into plot hooks.  He likes to say "WE HAVE TO BE AT THE CORNER OF 5TH and MAIN TODAY AT 3:47PM AND I CAN'T TELL YOU WHY OR YOUR MIND WILL BREAK AND YOUR MAGIC WILL DARKEN!!" or the like.

One of my group's PCs is part troll; his true form is much larger and uglier (while still within human parameters) than his guise. But we play it that the guise is really something that makes people think they are seeing something else, but somehow are subconsciously aware of the real thing.. so people react by giving him more room.  It's very common for him to squeeze into an elevator, and everyone else pushes themselves to the walls, despite him looking like a normal sized person.

By the rules, you also have to pay fate points equal to the refresh cost each time you cast it. 

Mechanically, you can start combining refinements and foci.  A basic 'Lightning Spell' is limited to refinements and foci from the wizard's Air (or whatever) element.  Make it a 'Summer Lightning Storm' and the wizard can also add any Summer bonuses.  So power and / or control may be increased by combining the two.  Additionally, a wizard who has Summer but not Fire can duplicate some of the things a Fire Evocator might accomplish. 

But, you'd have to pay for that refinement.  However, you could have both specializations at 2, which is way easier than getting one specialization at 4... so combining them might ber a power boost.  And, that could be combined with anything, really (summer rain, ect) so you technically could add summer's bonus to any element, making your best use to jack up the summer specialization as much as possible.   Of course, seelie magic doesn't give you a free specialization.

Dude, if fairies, vampires, ghouls, and lord knows what else can run around in human looking bodies, then it seems mean to deprive a player of that right.  All of my groups changeling characters start with human guise, and many take human form instead.  Human form is cool; they cant use their powers unless they hulk out.