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I solved Antoshka's puzzle in three ways. I also tried a fourth, but Inteus stubbornly refuses to use pathfinding before level 7, and that's when the battle in the garrison takes place, so pathfinding is no longer useful.
1) The Grail goes to the Flux. Despite the 60 mana cost for visiting the Flux, it is more than compensated for by visiting the oasis via the city portal. Accordingly, Inteus solves his logistical problem with a margin.
2) Inteus collects the gold, and Mephala is bought. She takes the Grail from Mullich, and she has enough moves to capture the Flux and exchange troops and artifacts with Inteus several times, which means Inteus saves 50 mana from Mephala capturing the Flux and barely makes it to Valeska. The Grail can be placed in a dungeon for fun.
3) Mullich collects 5000 gold, and Mephala is bought. Remaining with a centaur and visiting the oasis, she is able to dig up the Grail at the right point (move 2100/2100). Mullich places the Grail in the castle, and Inteus saves 50 mana on the pile of gold and barely makes it to Valeska. The battle for the Flux is significantly easier.
I don't rule out that there may be other solutions. Overall, it's a very interesting map, there's a lot to think about, and for 2005, it's really great, but by modern standards, it's not particularly outstanding in terms of difficulty. Two medium battles (harpies and the Flux) and the rest are easy.