This is a repeat of the Ashes mission from the campaign (apart from the initial portion in the tunnel), but from the point of view of Anna. You'll have to snipe the Watchmen for your partner at the beginning, then 'spot' the young dark one for him as he runs through the valleys.
A text walkthrough would be cumbersome so I've included a walkthrough video of 7 of the bonus missions below. The Anna mission starts at
1:12:04