The Zelda tutorial gave me an idea, and I decided to try it out. I'd still be hard pressed to how to do it with custom buttons on some of the engines, but you can save that for a common event or something.
This is absolutely scriptless, meaning you can use it in any engine. Also, unless you're using method 2, you don't even need any variables or switches!
Method 1: One Hit-Kill / Battle Screen
Set a monster event to collision with hero (or the equivalent phrasing for whatever engine you're using). Now create a condition with an else condition saying if the hero used the command button (in some engines, it will actually be on a specific button pressed). The condition should look like this.
Condition (If Action Button is Pressed)
Animation (Sword1, This Event)
That is, if you pre-empt the monster touching you, you instant kill them, otherwise a battle starts.
Method 2: Damage System
I've seen some over-coded systems in use, leading people to conclude you need some really good engine to do this sort of thing, and all sorts of X/Y screen variables. You don't (unless you're making ranged weapons).
You need two parallel map events. One sets the initial monster HP variables, and ends with Erase Event. The other sets your life, and displays some sort of hearts/meter.
Other than this, just do the same as before, except reduce monster hp (and erase when it hits 0 or less) on hit, and reduce yours if they touch you. Super easy, which kinda makes me weirded out when I see complex code for hit detection. This was really simple.