Character idea from an anime

[Railak]

So I watched Konosuba recently, entertaining to say the least, and I know everything works on different rules in the show, but I've been wondering if it's possible, to build a character like Megumin, the archwizard, master of explosion magic.
Now if you haven't watched the show you'd probably think oh, just take explosion based spells. That's not really how she works, she has 1 spell, and she can only use it 1 time a day. The power of the spell is rediculous, think small tactical nuke.
So, the power of the actual spell, in d&d or Pathfinder, isn't really too much of an issue, I'm having troubles figuring out how to make it the only spell that she can prepare, and only once, for the day, have no spell slots at all for any other spells. Does anyone know of a way to like sacrifice actual spell slots for like metamagic or just increase the power of the spell?

[The Viscount]

For a certain reading of Arcane Strike you can kind of do this, although you have to channel it through a melee weapon. Turn all your spell slots into bonus damage on a single strike. Perhaps choose all but one, and channel them through a weaponlike spell.

[Doctor Despair]

There was an almost identical thread recently. (https://forums.giantitp.com/showthread.php?622650-Creating-Megumin-The-Strongest-CL-20th-(delayed-blast)-Fireball&highlight=megumin)

Mycontil's Last Resort (spell) is the most thematic way to expend all your spell slots to deal big damage; Troll Blooded (feat) would allow you to survive it.

Erupt with Subdual Substitution works super well, but doesn't actually empty your other spellslots.

Another major issue with both Mycontil's and Erupt is that... you don't actually get them until level 17 normally.

Maybe you could do the Elven Generalist trick to daisy-chain up spells to the 9th level spell? (https://forums.giantitp.com/showsinglepost.php?p=10197586&postcount=9)I can't recall if it actually works;I'd imagine there's some debate on it. With that said, it gets you Mycontil's or Erupt at level 1. They will probably kill you though. If flaws are in play, you can take flaws to get two bonus feats (one of which will be Subdual Substitution) so you don't die.

You can use school restriction to reduce the types of spells you can cast down to one school; maybe there's a way to reduce it to no schools?

You could also go the route that she isn't unable to cast other spells, but just refuses to. If you're a prepared caster, let her leave the spellslots empty. Maybe there's something you can convert all her other available spell-slots into.

Fireball with Arcane Thesis, Subdual Substitution, and any number of other metamagic spells is a valid option (e.g. quicken, twin, energy substitution, energy admixture, etc) for a more practical build, but would result in being able to use multiple explosions each day.

Edit: I suppose the biggest fundamental issue is that there isn't really mana in dnd. You can proooobably use power points as the closest aproximation though. Spell-to-Power Erudite could be good for this. You can convert fireball into a psionic power, then expend all your powerpoints on it. There's probably some combination of metapsionic feats that would let you spend all your powerpoints on one massive nova of a psionic power.

[MinimanMidget]

Powerful spell once per day on an otherwise non-spellcasting character? Did someone say dragonmark? I think someone said dragonmark.

[Rebel7284]

Cleric 1/Shadowcraft Mage 3 (early entry) or Cleric 7/Shadowcraft Mage 3

You DMM heighten Silent Image for that Mycontil's Last Resort.

Then you are out of Turn Attempts and spell slots.

Surviving the blast is left as an exercise for the reader.