@suicidemouse

@ElementInfo(name = "Session Info")
class SessionInfo(x: Double = -8.0, y: Double = 57.0, scale: Float = 1F,
            side: Side = Side(Side.Horizontal.MIDDLE, Side.Vertical.DOWN)) : Element(x, y, scale, side) {

    private val fontValue = FontValue("Font", Fonts.font35)

    override fun drawElement(): Border? {

        val font = fontValue.get()

        RenderUtils.drawRect(0F, 0F, 100F, 60F, Color(25,25,25, 128).rgb)
        RenderUtils.drawRect(0F, 0F, 100F, 17F, Color(25,25,25, 190).rgb)

        font.drawString(name, (9F + 16F).toInt(), 5, -1)
        font.drawString("Nick: " + mc.getSession().username, 4, 20, -1)
        font.drawString("Kills: ", 4, 30, -1)
        font.drawString("FPS: " + Minecraft.getDebugFPS().toString(), 4, 40, -1)
        font.drawString("IP: " + ServerUtils.getRemoteIp(), 4, 50, -1)

        return Border(0F, 0F, 100F, 17F)
    }
}

You need to add the attack event, save the entity to a variable, then you need to add any event that is repetitive, for example onUpdate. You check if the world is not null and if the targeted entity is null or dead. If the player is dead, you increment the kill variable you have by 1, then you nullify the variable that keeps track of the entity you were attacking. Finally, you add the world event, that checks if there was a world change, so you can set the kills variable to 0. Hopefully this doesn't blow your brain and it shouldn't, because the client has these events available to you.