The most difficult alternative

Griffins, phoenixes, and mantises are a versatile combination, but losses in battle are inevitable, even against an equally strong enemy army.

There are no losses there :) Except in battles against level 4 neutral monsters, when there are many of them... well, more than 10. And even then, only at the beginning. For neutrals to kill even one phoenix with attack and defense parameters of ~40, they need to try very hard, and considering healing and various buffs like snake strike, luck, bless, armor, and regeneration, you need to be very inattentive to lose even one phoenix. So, even two such birds (without griffins and mantises) can break through any armies of levels 1-3 on the master difficulty. As for water elementals + archmage, this is very expensive (you need another 6500 gold for the portal), and there should be at least three archmages, or a well-developed archer.