[Phanx]

Not tested at all, but this should work:

1. Create a folder in your AddOns folder and name it "SimpleExecuteAlert".

2. Create a text file inside that folder and name it "SimpleExecuteAlert.toc". Inside it, write:

## Interface: 40000

SimpleExecuteAlert.lua

3. Create another text file in the same location and name it "SimpleExecuteAlert.lua". Inside it, write:

-- ***** BEGIN CONFIGURATION *****

-- Set the health percent below which to alert you.
local PERCENT = 0.35

-- Set the alert message to show you.
local MESSAGE = "Execute Now!"

-- Set the interval (in seconds) between alerts.
local INTERVAL = 10

-- ***** END CONFIGURATION *****

-- Store the time of the last reminder
local lasttime = 0

-- Create a frame to watch for target changes and health changes.
local f = CreateFrame("Frame")
f:RegisterEvent("PLAYER_TARGET_CHANGED")
f:SetScript("OnEvent", function(self, event, unit)
	if event == "UNIT_HEALTH" then
		if unit ~= "target" then
			-- We don't care about this unit.
			-- Quit.
			return
		end
		-- Find out how much health the target has.
		local current, max = UnitHealth("target"), UnitHealthMax("target")
		if current / max <= PERCENT then
			-- The target's health is less than or equal to 35%.
			if GetTime() - lasttime >= INTERVAL then
				-- It's been at least INTERVAL seconds since the last alert.
				-- Show the alert message in the UI Errors frame.
				UIErrorsFrame:AddMessage(MESSAGE, 1, 0.6, 0)
				-- Store the time of the alert.
				lasttime = GetTime()
			end
	else
		-- Don't need to check if it's PLAYER_TARGET_CHANGED, since
		-- that's the only other event this frame listens for.
		if UnitExists("target") and UnitCanAttack("player", "target") then
			-- Target exists and is attackable.
			-- Start watching its health.
			self:RegisterEvent("UNIT_HEALTH")
			-- Reset the time of the last warning, since this is a different target.
			lasttime = 0
			-- Immediately check the target's health.
			self:GetScript("OnEvent")(self, "UNIT_HEALTH", "target")
		else
			-- Target doesn't exist or isn't attackable.
			-- Stop watching its health.
			self:UnregisterEvent("UNIT_HEALTH")
		end
	end
end)

4. Change the values at the top of the Lua file as desired.