This achievement can seem really hard to get. The clips are reset on midnight GMT of 25th of every month. If you want to get the achievement you should try and do it around this time. Here is what you need to do:
1. First check how many downloads you need to get into the top 20. If it is just after midnight (GMT) on the 25th it should be less than 50.
2. Create a short clip so that it is quick to download. Share it.
3. You need a second gamertag. Create a new silver account or just create a free trial Gold membership.