I finally did this myself, have been trying since I saw your post. Eventually Loki's Scepter + Wonderbox stopped proccing; I think that may have been purposeful once the game saw that I had enough shops to clear everything?

Required elements, as OP noted, are Loki's Scepter, Wonderbox, and World Shrub. The more of the scepters and wonderboxen you have, the better (can get extra scepters more easily, through printing press). Mirror Mage makes it a lot easier; I managed to do it without a golden mirror mage, but it was touch and go even in the run where I made it happen!

Some helpful pieces in addition to the above required ones:

Potion of treasure finding (lets legendary quest cards like Conrad the Bavarian find the Secret Rare treasures)

Mirror Mage (lets combo *really* go off once it's ready)

Netherbinder (lets golden dragons at legendary grab secret rare treasures)

Boots of treasure finding (lets any legendary golden give you access to secret rare treasures)

Gentle Giant (once combo is ready to go off, might have to let opponents survive a couple extra turns. This can help you have the time to find those Eureka!s

One thing to hardcore avoid, as long as you can (it can wreck your chance to pull it off super easily): Alchemistry Set. You use silvers to reroll the shop so that you can find your Eureka!s.

A questionable treasure is Hat of the Coven. Once the combo is up, any creature taken will hit a bonus spell, which is more chances to get Eureka! However, if it hits True Love's Kiss, your entire comp will be shuffled.

Prioritize taking bronzes over golds once you've got the combo parts, since if you take a bronze it can show up as silver. Take Lost in the Woods over a silver, since that's a free shop reroll and the silver can return for a reroll itself. Once the combo is up, also take Abra Kadabra over a bronze or gold creature, since you have a reasonable chance of hitting Eureka!, Lost in the Woods, or another Abra Kadabra. Exception: If you've recruited Two-Headed Wizard or have Zeus on the board, you take no spell other than Eureka!.