Okay, playing this NPC as a caster who is smart but doesn't possess metagame knowledge or is necessarily a tactical genius. They're on their last legs, but so am I, and I can only reliably take down one of them. It doesn't matter which one, the others will be enough to finish me. So with the numbers against me my priority is escape. And thus the real question is, which of them is it best to take off the board to allow that?
I would say the best one to remove is the Fighter. They're the melee opponent who can tie me up with OA, not to mention enabling the Rogue's Sneak Attack. If I take out the Fighter, I can move first and then Misty Step, making it easier to break line of sight and begin my escape. Sure, the Cleric can pick them back up, but that buys me time. Maybe even a full round's head start, depending on turn order. The Rogue is fast but isn't as much of a threat if they can't Sneak Attack. The Wizard might be a danger, depending on what spells they have, but depending on the terrain I might be able to make it hard for them to catch up.
Dropping the Fighter, even if it's temporary, is the best bet for being able to make an escape.