I think spiders (without riders) MR is a bigger problem than militia for Machaka.
I'm fighting Machaka (who had a bless strat, so is rather forced to use his sacreds) with Arco in a game, and all our big battles end with me gaining half his hunter spiders as slaves -the other half being usually killed by his troops-
I suggest to give hunter spiders 10 or 11 base MR, even when the rider is dead. Actually Machaka is the only bless nation I know having so vulnerable sacreds against astral (of course he could have used an astral bless... and ended with 10MR on spiders, but I doubt it's really worth it).
Actually it's a good operation for any nation able to use enslave mind to attack Machaka, good way to gain a lot of free powerful spiders (eventually slavery as a reason to attack blacks may be seen as thematic
).