Z-wave momentary switch using Alexa

You should be able to do that with Webcore, although smartthings does not guarantee that instructions that are less than one minute apart will be executed. But there are quite a few people who are using sub minute communications in Webcore, Pistons (a “piston” is what webcore calls a rule).

See the logic FAQ (this is a clickable link)

If you’re willing to have the commands at least one minute apart, you should be able to use the “power allowance” official feature. This allows you to set a switch to always turn itself off a specified time after it is turned on. But the minimum time is one minute.