Even though only one name is written in the Death Note, if it
influences and causes other humans that are not written in it to die,
the victim’s cause of death will be a heart attack
There are three possibilities and chosing between them is speculation. None kill the boss. As Madara said, you could merely include the name of the Boss to skirt this. Lets say the boss, therefore, goes by the identifier B to avoid this. Lets say John Zerto is his bodyguard so his ability to obtain a gun or find B is trivial.
If you say "John Zerto dies of burns after killing B by shooting him" then John will just die of a heart attack.
If you say "John Zerto dies of burns after shooting B" then either John will just die of a heart attack or he will non-fatally shoot B before burning to death. I believe it is the former.
If you say "John Zerto dies of burns after trying to kill B by shooting him" then either John will just die of a heart attack, he will non-fatally shoot B, or he will attempt to shoot B but will inevitably miss every shot (or won't get any off). Of course he burns to death after either of the last two options. I believe it is the first again but this is speculation.
The only thing I know is that any of these options result in John Zerto dying and B living. I include here the possibility that B could be maimed but this can be logically excluded by assuming Kira is smarter than you. If he could mentally disable L by having a soldier non-fatally shooting him in the head, he would have. He would not have missed this possibility and, therefore, it does not exist. We can extend this to assume that extreme phyisical harm is prevented.
Note that this all must mean the universe of Death Note is either deterministic to some degree or the death note is intellegently moderated. The book/death itself knows if action A will result in someone else's immediate death or someone makes that call each time looking at the situation.