How to beat enemy unit? Demo

So in the demo, heading north, there’s a fight for a fort.

It is defended by a double shaman backline and a double glad frontline.

Easy enough to attack it, but the shamans weaken everyone so I’m doing next to no damage.

Even with a thief frontline who prevents a glad from healing, I still almost timed out on the mission even with an assist mage helping with damage. How should this be approached?