Eh, pure fireball strategies are kind of ridiculously overpowered right now (albeit completely ineffective versus magic immunity - and Counter Fireball, but I repeat myself). They're also quite reliable, since you either have enough MP resources to pull it off, or you don't. In this case you do: as long as you're an Elf or Gnome, take full advantage of Mystera's boons, and acquire at least one or two of the related items, it should be pretty straightforward - much less of a brain teaser / scum fest than, say, Thief or Paladin Gold.
On that note, I think I'd personally rate the difficulty of gold challenges something like this, from easiest to hardest (based on my own experience, of course):
Berserker (once you figure out the trick)
Thief (unless you get screwed by the dungeon layout, which is often)
Assassin (if you find a way of bypassing the sub-boss other than playing a Goblin)
Fighter (though I haven't taken a serious stab at it since the update - as the only (?) gold challenge that allows/requires preps, I'm not keen on spending a whole lot of my limited funds on it)
Bloodmage (seriously, is this even beatable?)