If you manage to survive till the 30th minute in any of the four stages in Vampire Survivors, the Reaper will appear. More often than not, your character would be one-shot while the stage is considered as completed. To defeat the Reaper in all four stages, there are prerequisites (see bottom of this post). You don’t have to follow a specific build. You just need to follow the strategy and the tips below. An OK build may take longer to down Reaper. But you should be able to get the job done within one minute. This guide also works very well against RNG. You need a bit of luck to get the 2 items you need but there are ways to mitigate this.
PS. This guide assumes that you have the knowledge and experience in surviving till the 30th minute.