What is the best way to kill the current ruler?

There is little else you can do. It is after all just a chance and a small one at that.

The simple act of making your ruler a general already significantly increases death chance(about twice as high). Battle do so as well. But the actual battle length isn't all that important. The longer a battle lasts, the more likely death becomes, in the end it evens out, as in the time a longer battle takes, you can do several suicide runs.

But even so, you are looking at chances of around 1%. Purposely killing a ruler or an heir simply is very difficult.

Also keep in mind, that a ruler/heir death in battle is a -1 stability hit, so if your ruler dies, you get -2 total (-1 from death in battle, -1 from ruler death)

There is another way to give your ruler a chance to die not mention in Dulkan's answer.

You can take a cog with a single troop lead by your ruler. Find an ocean region that has no supply and will incur monthly damage to your cog. Let the cog sink. There is a chance for the ruler to die as well. This is useful to avoid the -1 stab hit from "battle".