Walkthroughs for custom maps.

Yes, you're on the right track. On 6.1.1, the purple faction will become available, and you'll need to eliminate it, then rescue the hero with the Grail on the cross before the purple faction dies without a castle.
After that, I dealt with the pink faction and am now stuck in a situation where either the strongest hero ends up on the Grail's cross, or I just wait for the orange faction to arrive =(

Added after 37 minutes
Near the oasis, there are prophecy cards and lucky dove artifacts. With a cheat for infinite turns, you can reach these locations. However, I don't know how to accumulate that many turns.