I've tried, I don't attack downed players. The general flow is
Seemingly well-balanced encounter (although higher in difficulty than others because boss monster)
Stab-wizard (not always a wizard) goes down first, boss shifts focus to other players.
Other players panic as the death saves rise, and will stand right next to the boss monster trying to heal the stab wizard, taking huge damage and dealing none
Next player goes down, snowball, TPK
Unfortunately, none of the players are really well versed in the system (and don't really want to learn beyond in-game learning), so even though they'll put two characters right next to the boss, they'll never flank etc unless I explicitly remind them at that time.
I appreciate that the other players feel a sense of camaraderie and won't abandon the other player, but I'm not going to have a dragon just suddenly decide their going to do ranged attacks when there's someone standing directly in front of them that is actively distracted doing something else.
I've tried the typical advice of minions with a lower level boss, and while the individual monsters are weaker, when the stab-wizard goes down the party gets fucked by the action economy.
Sometimes I'll do what I did last time, which is just debuff the boss monster to a significantly lower level (moderate encounter level) but then (as happened last time) you get one lucky roll and it's dead at the start of combat and you're like "fuck that's not scary or fun"