When you make a ruling, explain your justification out loud. If you're targeting a random person, tell the players the stakes before you roll. "OK, evens the wyrm goes after Faust but on odds it'll go for Drena even though she's on the ground."
Saying what you are doing at all times is the absolute best thing you can do in my opinion, because it makes it clear to the players what your intents are and promotes honesty.
I would spice it up with an in game description, such as the dragon flicking his eyes between the two and saying "The wyrm flicks his eyes between the two characters. Evens for Faust and odds for Drena (rolls). The wyrm dives to the ground at Drena."
In my combats I will have monsters convey reasons for what they do, such as a melee character saying "A challenge!" when focusing on the fighter over the bard or "Out of the way!" if they take a swing at the Bard before focusing on the fighter. Goblins screaming in fear before disengaging if they survive a hit, or even a humanoid yelling "Fall back!" to make it clear why they are making a coordinated retreat and allow the players to end combat some other way than just killing everything they come across.