The most challenging maps for you.

Chingis, starting with the Conflux in "A Beautiful World" is indeed probably the best option: it offers a favorable geographical location, and Phoenixes appear relatively early and in greater numbers than level 7 monsters of the opponents, and the Book of Air Magic (although it's not necessary for a speedrun). Also, if you acquire the best spells in the mage guilds, you can win quite quickly.
Detailed information about the Sphere of Vulnerability in the Physical Might game is provided in a separate section, as there are several non-obvious nuances. In most cases, it "cancels" the immunity of creatures to magic - both your own and your opponents'.