fear i believe you can affect target stats as it is, but i'm unclear if you can affect ai, i do know that some of the AI types run if they no longer have magic points...so try this backward method out.
when you cast the spell it changes the value of a global,this changes teh conditional of an npc spell to true, they use the spell it squanders their magic points they are then forced to run.
or delete and replace the npcs at their actualx&actualy; these would be set to flee, i'm using something similar for something i will be showing very soon, non-showcase related.
nova for the npc is easy, as they only have one target
( correct me if i'm wrong they can't cast on their fellow hostile npcs, can they? Even if they can they can cast the spell multiple times very quickly )
so basiclaly the spell would just be a graphical effect, to keep teh effect from firing more than once, use the 'cast' conditional event to launch the animation;
add 1 to gb_novaSpell
use a conditional does gb_novaSpell equal 0 if not End Event
else using the stamp control with the npcx, npcy coords launch the animation
additionally you could count to see how many hostiles npcs are in the conflict, ( its a predefined system global something like CounterHostileNPCs )and when the count equals the total amount, then fire the animation.
invisibility, well it seems that oyu might be deleting them and replacing them with versions with no LOS,
hmm, i can think of a way to get invisibility but its nt efficient, one would involve placing teh player inside a container and unlocking the camera and having them control it from inside their holding pen and when the spell expired place them at camera x,y
the other way is to enable a spell for them one at a time the spell triggers their death event, this will divulge their location because they are then the current acting npc and in their death event place a new npc in teh location, this will not work at fast speeds i suspect, and is subject to error... :: shrug ::.
another idea is that they can choose a strategy for entering;
stealth or through the front door (ShinRa HQ, FF7...i came through the front door 
)
this would allow you to create two versions of the map, one with blind characters no LOS and a char with no collisions sphere, which means you'd have to intentionally run into a guy for him to realize you are present.
such an event would be placed in the initialization event, similar to the little girl that follows you in the demo game.
The problem with switching to a location on the fly mid game is that it resets the npcs and their locations, health etc...this works if your game is like Zelda action/adventure...and actually works for an rpg as well, its just that you might anger some people because they might currently be struggling with defeating the npcs and seeing them all restored could be aggrivating.
a see invisibility spell would be something they cast to bring you back to teh current camera coords, ie if you saw them casting you could make the camera run away to avoid being in range when revealed...the problem with playing with the camera is the camera doesn't avoid walls and it is possible to get trapped inside of an area of no access...all in all i might just stick to raising the player's evasion stats, ones that make him faster and harder to target. that and switch to an animation that's got not collision sphere...this is intangible which is far easier to implement than invisible.
karma's the only one you can use, you can seriously think out what you want to use some stats for and if the value isn't relevant to your game then you can use it, ie if noone uses cold protection/attack then its yours to manipulate, and you could multiply it by 100 everytime to get an integer number.
but unless you want them checking dialog boxes there's no other way to display the contents of a global directly, without using graphics and a loop to check the value and alter the graphics.
the answer exciting spells = advanced work, for each one of those spells you have to put a system in place to manage and execute them, and make sure they can occur synchronously..making the engine do extra math slows it down.
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... bwahhaaaaahaaaaaugh fooooolish sammurai.
I'll just bookmark this thread to check it out later