Missy Peregrym (Maggie Bell) was pregnant with her second child during shooting of this season and was written out of the show during this episode in a way that allows her character to return at an undisclosed later date.
Despite what OA said it doesn't require any expertise to know how to use and position a claymore. A claymore is a directional anti-personnel mine, it is packed with metal balls that explode outward in one direction from the front of the device at about 1,200 meters per second (3,937 ft/s). It is designed to be quickly and easily deployed, it even has the words "FRONT TWORDS ENEMY" stamped on the front, all the user has to do is point the mine in the direction the intended target(s) will be coming from and arm the fuse. They are usually activated with a trip wire, but newer versions also have motion sensors that activate the mine.