In the case where there are no valid targets on which to focus, because there are no valid hexes a monster can attack from (i.e., they are all blocked, occupied, or there is no open path to reach them), regardless of the number of hexes it could move, a monster will not move or attack on its turn.

It will perform any of the other actions on its ability card that it is able.

In the example, the guard will still "strengthen" itself. Even though it won't move or attack since it can't focus on any character.