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If you're going to withdraw troops from garrisons, do it right before Biara appears. I withdrew troops from the central garrison (which she passes through) one turn before she captured it. The mission isn't easy, but if you don't lose too many troops (especially archers) and capture the building with archers with minimal losses (it's to the west of the city, if I'm not mistaken), the battle with Biara won't be too difficult. The Necropolis faction tried to capture the garrison three times – they succeeded on the third attempt, but didn't go any further. I, in turn, tried not to approach the border too closely, and they didn't attack again. Biara arrives in about a month (I don't remember exactly).
In general, try to make sure that the Necropolis faction doesn't capture the first garrison on their first attack. When they do capture it, don't try to recapture it or stay near the border. Instead, rebuild your city, attack neutrals, collect artifacts, etc. And as soon as Biara arrives, withdraw the garrison that she's going to pass through. If you've developed and fought well, you'll have enough troops to defeat her without too much trouble.