That being said, I think it's time for the Trollgod's rant on the nature of magic in Trollworld.
Magic on Trollworld is the ability to altar reality to conform with what you want that reality to be. This is done by channeling a magical force, kremm, emitted by the planet itself, and stored in minute qualities by living beings. Reality resists change, hence a disciplined will is required to change it. We simulate the use of a disciplined will by requiring a saving roll on INT at the level of the spell being cast.
In 5th edition and earlier, wizards did not channel kremm so easily, They spent their own personal strength/lifeforce to channel kremm, and it was possible for a wizard to overextend himself and die from spellcasting. The use of one's personal lifeforce to cast magic is one or two orders of magnitude greater than the mere channeling of the world's kremm, and thus it affected everyone.
Reality resists change. That is why someone with greater kremm is immune to change/damage from someone with lesser kremm. Imagine two wrestlers shoving each other. The bigger, stronger wrestler will aways push back the smaller, weaker one, altho, to do so, he must use at least as much energy as the smaller one. This is not to say that the smaller one can't deflect or trip the bigger one. In wrestling, it would be done by pulling instead of pushing, using the bigger guys strength against him. In Trollworld sorcery, it is done with spells like Shield Me or Protective Pentagram.
Kremm resistance as written into the 7th edition rules is actually a simplification of a Resist magic spell that I was originally going to put in there. Since all wizards are sensitive to the existence and flow of magical kremm energy, they sense spells being cast upon them--those forces must be gathered and launched, something like throwing a snowball--and they would have time to at least try to create their own defense. Magical defense would be one of the first things taught to young wizards in their training. Everyone would have it. No defense is perfect, however, and a weak defense, while better than no defense at all, can easily be overwhelmed by a strong attack. Likewise, a strong defense will always beat a weak attack.
Rather than slow the game down by having both/all players declare attacks and defenses, I used kremm resistance to deal with it automagically. :) Weak attack vs. strong defense = kremm wasted by both parties. Strong attack vs. weak defense = success. Assuming that the weaker defender would actually try to stop the stronger attack, the loser in that case would lose all his kremm at the same time. It's better to have no magical defense than to put up a weak one that costs all your remaining power. If the weaker opponent survives the initial attack, and maintains his kremm, he may attain the superior position and strike back.
Since everything living has some ability to store kremm, the problem of one against many becomes the problem on one overcoming many. Logically to directly affect several people at once with a TTYF spell requires that the caster have more kremm (greater WIZ) than all the targets combined. Or, perhaps the spell could overcome the first person, and then the second person, and then the third target until finally the sum of the targets' kremm is greater than that of the caster and it fizzles at that point.
What you need to understand in Tunnels and Trolls is that magic is not chemisty. If you take a flamethrower and spew fire at several foes, that fire has real existence and burns everything equally. If you cast TTYF and spew destructive force at your foes, that force only exists in that part of reality that you were able to change through force of will.
Too complicated for a fantasy game? Sorry bout that--it all seems quite logical to me.
This is why wizard fighting multiple foes do not tackle them directly. It is much easier to alter reality and use the world against the foe. Do a slush-yuch spell. Do an avalanche spell. Do a lightning storm spell. (I know, you haven't seen lightning storm and avalanche spells). I suppose I'm going to have to do a book on how to fight effectively as a wizard. Very few wizards get any tactical training, and I think that's why they don't do well in combat situations.