Ahriman
Tyrant
How many people use workers or slaves to defend stacks with weak valuable units like spellcasters? I'm playing at a lower difficulty level, so I usually don't need to do this
I do this all the time. Lightning elementals also work great. Or summoned skeletons.
I wish that the Defender promotion would give the unit higher defender status *only* against Marksman attacks. Its useless when you can't use the better defenders to block normal attacks, and the Defenders to block assassins. Like Digit says, normal units take out the sacrificial defenders and then assassins are free to kill the mages, which are irreplaceable because of the time to generate XP.
I can't help but wonder if the slow-XP-accumulation mechanic and level requirements are bad design choices. The AI can't handle them so magic is effectively a player-only phenomenon. The AI can get research bonuses to build high-tech units, but it can never preserve its units long-enough to get high XP ones. The concentration of power into such slow developing units is also a slippery slope; as long as you don't lose your powerful units, you can make them even more powerful and win battles and such at no loss, but if you lose them (eg because of some uncounterable assasins or something) then they are too hard to replace.
This, along with the power of the Agriculture civic and the doubling of the value of combat promotions, are the main reasons why FFH is so much easier than vanilla civ.