It doesnt matter which focal points you use, as long as you are in a good position to hit the boss with it. However, If the boss is a few distances away from you I would typically use the the dash skill + focal point (right click right after) to propel myself to the boss faster in a short amount of time while getting a few hits in. If you still cant reach the boss after using the dash skill, hold down R for that extra dash while chaining into the focal point. The dash + focal point skill can also be used for dodging attacks as well.
A good combo for chaining focal points if you have enough sp would be: Focal Point (active) -> Dash skill + focal point (right click right after) -> Holding R to dash then focal pointing (again pressing right click right after) -> Repeat.
LS Fiona's main DPS is her enhanced counters that chain into arma kick, focal point combos, deflecting, and the regular LLLR.