So my 18th Level Wizard walks into the Frost Giant Jarl's Throneroom and casts Meteorswarm and use all 9 uses of my meatmagic: 4 for Energy Admixtur (might as well in add fire), 3 for Maximise and 2 for Enpower.
Hmm ... the Maximise and Enpower means 4 Fireballs that each do 36+3d6 Fire dmg and the energy admixture I am a little fuzzy about [is it a) another 36 fire dmg to each b) 6d6 to each c) 36+3d6 to each or d)9d6 to each] Any way: if its option B [and i am picking it only cause it does minimum avg while i believe it should be option d] we are talking about (36+9d6)x4 or an avg of 270pts of area damage [we won't even throw in vunerability].
Now some of you might like this, however I have a problem with someone spontaneously casting what amounts to a Level 17 Spell, because you cant memorize it. Even if you have the epic level feats that that lower the cost of metamagic (you have to take it three times by the way), the best spell that you can admixture + maximize + enpower is a 6th level spell and not a 9th level spell.
So, in conclusion, I believe Metagic mastery should either be limited to adding only 1 metamagic to a spell or you have to cap the amount of metagic added to a spell based on the highest level spell you can cast [that is, that the 18th level wizard can only enpower+admixture+maximize a lvl 0 spell as theoretically the metamagics involve are a +9].
