Heretic of The Seventh Circle
I combined the two.I think 13th Age handles the Instakill thing best. Don't give it a hit dice limit - give it a hit point limit. That way the Assassin's iconic ability isn't useless against the boss monster, They just can't use until it they bring the monster within range.
In tier 1 you get
Lethal - when you deal damage to a creature and reduce its hit points to a number equal to your Assassin level or lower, you can reduce the target’s hit points to 0 instead.
this is pretty small by itself, but does get upgraded eventually to level+intelligence score. Which is still not huge. It’s 40 max at level 20.
Then later (don’t recall the level offhand) is Death Strike, wherein you can instakill a target that is CR x or lower. I think about half the cleric CR threshold?