There's also the problem that a Specialist in DnD isn't gonna be as good as a generalist who is equally good against every monster type.
Being specialized in a specific terrain and a specific class of monster can be cool and flavourful, but it means there's a strong chance your adventure could bring somewhere where your specialty is utterly useless.
This has long been a problem for Rangers, and even being able to choose multiple types of foes over time still puts you in a "am I average in this battle or super good?". Smite has similar issues, though at least many foes were evil. Currently, Paladins don't even care about alignment (though they get bonuses against undead and fiends), so they are even less restricted than in previous editions.
If I really wanted to capture this feel, I think I would do something like this on a subclass level: at level 3, choose a creature type. Once per turn when you hit with an attack, you deal 2d6 additional damage to creatures of that type, or 1d6 to other creatures.
At level 7, you choose a second type of creature. Once per turn when you hit with an attack, you deal 3d6 additional damage to creatures of either chosen type, or 2d6 to other creatures.
At 11th level, you choose a third type of creature. Once per turn when you hit with an attack, you deal 4d6 additional damage to creatures of the chosen types, or 3d6 to other creatures.
At 15th level, you choose a fourth type of creature. Once per turn when you hit with an attack, you deal 5d6 additional damage to creatures of the chosen types, or 4d6 to other creatures.
The bonus damage is only once per turn, and at best, half what a Rogue does, and it's only slightly better against your chosen foes, akin to how a Paladin's Smite is only slightly better against undead or fiends, but it captures the flavor that you are a slayer of certain monsters, but your general knowledge applies to most creatures.