Also now that you have played a session, I would highly suggest reading a good chunk of the "aspects" and "playing the game" sections.  Reading them again after having hands on time makes a lot of things make more sense.

It might be an idea to consider giving your big bad shagnasties Physical immunity against mortal magic, like Ogres. This forces the Wizard to do other stuff than just nuking them (maneuvers that do not directly affect the monster, clearing out mooks, etc.), allows the other party members a chance to shine while taking them down, and keeps things interesting.

Exactly, you just have to get clever.  Though I would suggest NOT falling back on the "it's immune to magic" tactic too often.  It becomes very obvious to the players that you're deliberately trying to hamstring the wizards.  Players start to not have fun if they feel they are constantly being handicapped.

Variety is the key.  For example my last session had my players in a cargo van trying to escape a pack of Black Dogs.  At the bottom of every exchange the npc driver would make a driving skill roll.  The group needed to accumulate 10 shifts of movement to escape the dogs.  At the top of every exchange I would roll to see how many new dogs joined the fight.  They were easy to take out, but they would never stop coming.  More would join every exchange.  I also gave the van a stress track and consequences, so the dogs could attack the van directly.

This forced every player to get very creative, not just the magic users.  As it didn't matter how many mooks were killed, they had to play defensive and really work maneuvers and declarations to keep the vehicle safe.

Simple answer, clever GMing.  Is the group always looking to the wizard for answers?  Design scenes or entire scenarios where other players assets are forefront.

Wizard relying heavily on summoning up beings for information?  Make the price for answers too steep, or make it very clear that there are consiquences for getting in bed with demons and their ilk by having it come back to bite him in the ass.  Or have a summoning go wrong and the creature gets loose and people get hurt or even die.

Follow what the rule book says.  Magic doesn't make things easier, if anything it makes things more complicated.

Taran makes a damn good arguement for emmisary of power.  There are a few episodes of the show that play into the "spirit of the slayer".  One where Dracula talks about how the slayer power is "rooted in darkness" "a darkness that rivals my own".  Which tracks later on in the show when the origins of the slayer are revealed.

Also the first episode that The Primal (the spirit of the slayer essentally) is in.  It gets pissed that Buffy allows her friend to tap into the spirit for a spell.  That the slayer "walks alone" and "no friends, only death".   

another one where Spike talks about how he killed two slayers.  Saying that a slayer deals in death every day, that they are just a little bit in love with it.  That every slayer secretly has a death wish.

There is a box about "mundane spell effects" somewhere in the book. Simply put: a spellcasting character can do small spells without any effort. Making light, lighting the fireplace, generally, things that don't really have an impact on the narrative (Harry could flick a lightswitch or take some matches, but it is just cooler for a wizard to do it with magic).

If you are in a conflict however, that does not apply. So in your situation, the hidden character would have rolled a stealth "block", he is hidden. He probably tagged some aspects ("dark shadows") for example. Now the PC come in and you let them make an alertness roll to see if they can spot him. Since it is dark, the wizard might make some light, but since this is a conflict, he would have to do it as a maneuver. Maneuver spells in general last the entire scene, so you don't have to put extra shifts in duration. The new aspect ("light") now cancels out the shadow aspect, and since it is gone, it can no longer give a bonus to the stealth roll, which might be enough for the players to see the hidden character.

If you are casting against a darkness spell, the higher powered spell should prevail. A maneuver needs at least 3 shifts, but you can put in more to increase its power, so it is harder to resist it.

This seems to be a general misconception about magical maneuvers, unless RAW contradicts itself at some point.  Magical maneuvers do not last the entire scene, assuming we're talking conflict, according to the RAW.  YS 253 (pertaining to magical maneuvers) "As with other effects, you can pay an additional shift to make the effect persistent at the rate of 1 shift per additional exchange."

Though in the case of trying to detect someone in stealth, using a light maneuver then tagging it for an alertness roll (versus the npc's stealth) revealing the target.  If I were the GM I would say it doesn't really matter if the light aspect goes away next turn.  You've uncovered the npc from stealth, it wouldn't make sense that he could suddenly vanish once the aspect was gone, unless he has powers that actually let him vanish haha.

I would do it like that as opposed to a block or defense because the GM doing a roll of any kind basically tells the players that someone is lurking around somewhere.  Then they may start playing in ways that can break the scene.  Where as a general altertness roll made by the players doesn't force you to tip you hand quite so badly.

After looking at all the strength power i was wondering how you determine the maximum amount of weight ,in tonnes, kg, lbs etc , a character can lift. Ex, If someone had a Might Score of 10 how much weight could they carry.

Other people will likely have a different take on this than me.  But personally I wouldn't even think of it in measurements of weight or size.  Think of it in terms of difficulty ratings.  What would be the difficulty roll for a normal mortal to lift an iron gate?  A car?  A bus?

The number of shifts needed to succeed in lifting something doesn't change if you have a strength power.  It just makes it easier to reach that number.

I'm not sure I follow. How would this work exactly?

Basically when you start the scenario even you don't know exactly who the killer is.  You instead have say 3 different npc's that can be the killer.  Then during the game you decide who the killer will be based on the actions and ideas of your players.

Haha brilliant.  If he keeps with the character, he has no idea the world of problems that will come with that sword.  That's why I want to get a player for my campaign that has never read the books.  You can set them up for all sorts of things that a fan of the books would see coming a mile away.