Majoru Oakheart
Adventurer
If you've setup the set piece boss encounter and you have the boss there solo with no cronies...Heat Metal or not, that's going to be a meh battle. Not sure why a boss, wouldn't just escape with odds like that.
I'm not sure exactly HOW bosses escape in your games. One of the longest running rules for being a D&D DM is "Never have an opponent that you want to live get within sight range of the PCs" because once they are, there are about 100 different ways for the PCs to prevent them from leaving: Silence to prevent spellcasting, grappling to prevent them from running away, Hold Person spells, Entangle, or just the ability for most PC groups if they throw everything at a problem to do 100% of the hitpoints of most enemies in one to two rounds, often before they can even take an action to try to run away. That's in addition to the large number of opportunity attacks they tend to provoke as they try to leave melee range.
Running away often requires 2 or 3 rounds just to leave the range of the PCs ranged attacks. If the PCs chase, they often move faster than most enemies, so it's impossible to lose them.
The only possible way to get away is pretty much a teleport spell of some kind and given how high level that spell is, the trick only works at high levels. Even then, it only works with spellcasting enemies.
Basically, if an enemy fights the party, they are dead. Reoccurring villains only work if they mock the PCs from afar and act only through minions. It's been like this since the beginning. Dragon Magazine was filled with people writing in and asking "How can I keep reoccurring villains alive?" and the answer was pretty much always "Don't let the PCs get within sight range of them. There's no way to guarantee they get away."