Sneak to position your party, try putting down some oil surfaces to slow goblins or get them to use more AP going around, etc. After your opening grenade (or fireball, etc) to start the fight, you can add more oil or fire to the mix, engulfing anyone trying to go around.
It is possible that aiming for a foot / elbow, or the top of the head, etc, can get a clear line of sight, when most of the target is blocked. Since the line of sight is calculated from the middle (IIRC) of the model, it is also possible that the middle of one NPC has a line of sight to some part of a character, but the middle of that character does not have line of sight to any of the NPC (depending on positioning).
