Finding a focus for a monster can be tricky in certain circumstances, but it follows some very basic rules.

First of all, is the monster performing an attack on the ability card it drew? If not, pretend as if it is performing a melee attack (i.e. it wants to be adjacent to an enemy).

Next, find path using the least movement points to get within range (and line-of-sight) to attack an enemy. Note that the monster focuses on a target but the movement path is towards a legal firing/attack position hex (unoccupied, in LOS and in range) - its destination hex. It doesn't matter if the monster can't get to its destination hex with the movement it has, as long as there is a path to eventually get to the hex it will still consider this path the optimal path. The enemy/enemies that are within range (and line-of-sight) at the end of this shortest path are considered the "closest."

Side note on negative hexes (traps or hazardous terrain): negative hexes are considered obstacles when determining this path unless there is no path except through the negative hexes. The path can be ten steps long if the enemy avoids the negative hex and two steps long if it goes through the negative hex, it will still choose the ten-step path as long as it is available. Basically, monsters will move through as few negative hexes as possible.

In the case where the monster can use the same number of movement points to get within range (and line-of-sight) of multiple enemies (either because it starts its turn within range or multiple enemies or through some other situation), proximity (i.e. number of hexes they are away, not counting through walls) is then checked as a tie-breaker for "closest."

Okay, now, if there is only one closest enemy within range at the end of that path, that is the monster's focus. If there are multiple closest enemies within range (and line-of-sight), the focus is the enemy among those tied who has the lower initiative for the round.

Note that there may be some cases where a monster can't find a focus at all. In order to obtain focus on an enemy, there must be an unblocked path (no walls, enemies, obstacles) to a hex that it can legally stand in where it could eventually move to and target the enemy. If an enemy can't find a focus, it doesn't move or attack on its turn but it can perform any other abilities written on its card.