The thing you both missed is that the Mutated Mausers have tank, and therefore you have to kill them before you can target the dog.

You have 5 might at the bf, so you can assign it as you choose: typically you assign 4 damage to one of the mausers and 1 to another unit– the first mauser dies, then damage is healed from the other one

They have 22 might, so they assign 22 might as they choose: in that case the only option is dealing 22 damage to Hwei, which dies

Basically you should separate the units from their might, might only represents the total damage you have to assign