Your request needs more detail, and it belongs in Tutorials.
Under what circumstances do you want the monster to respawn? If monsters are events and you just want him to come back if you leave the screen and come back, then all you need to do is put an Erase Event command. If you want the monster to respawn on a timer, then you can use a switch which changes the event to a page with a blank graphic, make it a parallel process, and use wait commands (or possibly a common event and a variable if you need it to count even when you leave the map.
If you are talking about respawning the monster during a battle in the DBS, you might be able to use a simple invisible monster if it is a special battle, but if you want it to happen randomly you might need a script.
In short, give more detail. You might know what you're talking about, but nobody can help you unless you communicate that using your words.