I encountered Shaka attacking with Impis in one of my Emperor games where I was playing as Rome. I threw Legions in front as meat-shields, then a handful of composite bows to finish off his wounded Impi. I got pushed back to Antium, but once pushed back that far I held on by knocking out his siege units and using zone-of-control tactics with my remaining Legions to keep his Impi from attacking before I could hit them with the remaining composite bows and the city-bombard. As Rome, I had the Legion, but even without them, I could have also built pikes for about the same amount of cost and gotten no less utility from them.
The key to defending against Shaka is not to worry about the Impis, but to worry about the trebuchets. Assuming you had a decent force to begin with, his Impi will not be in full health when they reach their target city, and will depend on siege units to whittle the city down. This is especially the case as the AI rarely builds an army of only Impi, Composite-Bows, and Trebs; often, and as was the case in my Rome game, Shaka also sends swords, horsemen, antiquated warriors, and archers in the mix, meaning the force rarely is primarily made up of Impi.
I've played Immortal in BNW but not yet encountered Shaka in the game or two at that level. But on Emperor at least, he was reasonably containable (as opposed to G&K's Huns, whose Battering Rams render the normal composite-bow defensive force fairly useless).
The key to defending against Shaka is not to worry about the Impis, but to worry about the trebuchets. Assuming you had a decent force to begin with, his Impi will not be in full health when they reach their target city, and will depend on siege units to whittle the city down. This is especially the case as the AI rarely builds an army of only Impi, Composite-Bows, and Trebs; often, and as was the case in my Rome game, Shaka also sends swords, horsemen, antiquated warriors, and archers in the mix, meaning the force rarely is primarily made up of Impi.
I've played Immortal in BNW but not yet encountered Shaka in the game or two at that level. But on Emperor at least, he was reasonably containable (as opposed to G&K's Huns, whose Battering Rams render the normal composite-bow defensive force fairly useless).