In my opinion, the Archmagi are well worth the price and are by all means one of my favorite units. Having incoming damage reduced by 50% is well worth it. In most cases, with stone skin/divine armor + magic shield, your tank stack will suffer minimal losses. Let's not forget that the 50% damage reduction is absolute - it works regardless of the damage incurred.
Let us not forget that the Archmagi have a decent HP value, so they will not fall like butterflies against bosses. Their damage is magic (ranged) and physical (close combat), which gives you 2 types of damage to work with.
This goes a long way towards making no-loss victories. I'd rather have 1-2 support stacks that help me minimize the casualties than 5 offensive stacks with big damage that I will have to replenish all the time. It's costly (especially on impossible where income is reduced by 50%) and it's time consuming.
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