Challenge in Sandboxes

[NichG]

Isn't it just easier to create tomes at that point?

Wish to create magical items directly has additional XP costs, which then further increase the DC and drain. For +5 tomes, that's 11000xp (137500/12.5), which is +110 to the DC. I think the maximum bonus you get for taking as long as you like to cast (barring things like rituals cast over multiple years) is something like +35 (I don't have access to Advanced d20 magic right now so I can't confirm this, but actually that might be high). So at that point if you can survive the drain, it's better to just cast the five Wishes in a row. However, I think there are more clever things than that using metamagic (which just adds to the DC, so you can metamagic up Wish to your heart's content if you can pay the price and hit the expanded DC). So a Twinned Repeat Wish gets you a +4 inherent at the cost of 7 spell levels = +35 to the DC. I don't remember if that's how we did it though.

Spoiler: Slayers d20 casting Wish at Lv12 math

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Casting check is a Fort save, but there are various class features and optional modifiers you can use to fiddle with it.

Start as a Loremaster, prestige into (Slayers) Sorceror from Lv6 on. This means a base Fort save of +3, but also gives +5 to casting checks from a Loremaster secret, +7 to casting Wish from Sorceror levels. It might be worth fitting in two levels of standard D&D Paladin to get +Cha to saves and the good fort save, which would sacrifice one Sorceror level and one Loremaster level, but that definitely wasn't what this player did. For reference, that would give a base Fort save of +6, +5 casting from Loremaster, +6 casting from Sorc, and +Cha mod to the fort save.

For feats, take Great Fortitude (Slayers d20 variant feat, +4 to Fort saves but you have to eat like crazy), Spell Mastery (+5 to cast specific spells), Magical Blood (+2 to casting checks).

Coop-cast with another two casters to gain 1/2 their CL to the save each = +12

Name and Incant = +10

Use a 10kgp material component = +10

Use a focus = +2

Take drain as lethal = +10

So if you have (with items) a +5 modifier from Constitution and a +5 from Charisma and a +5 resistance bonus from an item, that gets you to a +87 to cast Wish in one round for the Paladin build, which has a chance of success (but failures are painful). I think there's probably some pre-buffing I'm missing here to close the gap a bit further.