General theory of one hit kill
cheats
When you attack something in a game there will be an equation that runs to determine what goes. This can vary dramatically between games but good things to look at are enemy HP values, attack ratings for weapons and characters, any debuffs (no ground attack against air sort of thing) or multipliers, hit detection and more.
If you want to make a one hit kill you get to fiddle with those. You can do it with various stats in RAM or you can do it by altering the game's code (which depending upon the system/cheat setup you might also be able to edit in RAM, more classic consoles will want the game genie).
The higher end hacker will probably fiddle with the hit detection aspect as they will probably find that most effective for the effort (if you can edit that then it will work, if you start looking at atk or something you might not get what you want).
The lazy or inexperienced hacker will probably first look at the other things mentioned and pump high or low numbers in there to achieve much the same effect.
I will skip the higher end thing and look more at the lower one.
For enemy HP you will want to find where it stores HP in the memory. Do note that multiple enemies on screen might be handled in multiple locations so to start with you will want to go somewhere where there is just one enemy on screen (or make it so) and start your searches.
Finding HP is a fairly classic cheat (get where you can see and manipulate it, do as little as you can other than to change the health value, search, change the health again, search, change health, search... until you find it or can brute force it. Some will use ineffective weapons, find enemies that regenerate health or have high health such that they don't die before you finish your searches -- while not impossible to go between enemies it is far easier if you have just the one.
Once you have this you probably want to find others for when there are multiple enemies on screen. Typically they will all be either one after the other, of if enemies have more complex stats then the enemy stat tables/entries will be one after the other.
You then will make a cheat of the form if health value is greater than 2 or whatever set it to 1.
Possible problem is if the enemies have a low health behaviour then it will be running all the time. Solution to that is either change that behaviour, use another approach or maybe find the maximum health value (it will tend to be right next to it) and change that as well.
ATK values if you don't want to edit the ROM work much the same. Find where they are located (level up if it is a thing here, equip different weapons, possibly change character order in multiple character parties, Once you find it set a ridiculously high value. If you have a stat based chance to miss then you might have to sort that as well, also even full value ATK might not be enough to one shot a particularly hard character.
Debuffs and multipliers are harder. If there is a way to boost damage do that, figure out where it stores that in memory and set it with a cheat (hopefully it will just be a simple flag in the memory somewhere). Normally we look at this sort of thing for moon jump, monster away/no random battle
cheats and
When searching for
cheats in most emulators and the like you will be given a few options
Search for specific value (if it says 100 gold you probably have 100 decimal somewhere in memory). Not all things with specific numbers will match but if it does then it makes things far quicker.
Things which changed since last search
Things which did not change since last search
Things below a certain value
Things above a certain value
Things which increased since last time
Things which decreased since last time
Number is between this value and this value (possibly also the reverse of not between this value and this value).
Most of them are fairly obvious. If you are finding HP to make an infinite HP cheat then if specific values don't work then getting hit will lower it, taking a potion will raise it. In the event that it is a float value like mentioned it will trouble the simple search parameters so you will instead have to use the changed/not changed and take a bit longer to narrow things down.