There's no right or wrong way to beat writer's block, but there are tricks that might work for you.
One way to beat writer's block is to write the end first.
Your writing process doesn't have to be linear. There's no reason why you can't write the end first and the start last as long as you get the job done!
If you follow the structure of a 'standard' radio commercial, you will usually have the call to action at the end.
For example: "... Hurry, this sale won't last long at Shop Name, 123 Example Street. Terms and conditions apply."
That's 5 seconds out of the way, so all you have to do is work out what to do with 25 seconds. In a way, you're chipping away at a 30 second commercial five seconds at a time.
This method also gets your brain ticking and you get in the mood to write.
I've been using this method for years when writing radio commercials, blogs and other forms of writing and it was awesome to see that Dr Sirini Pillay included it in his book 'Tinker, Dabble, Doodle, Try' - that means that it's backed up by science.