You could do something like:
SearchClient.Instance.Delete<object>(x => x.GetType().Name.Exists() | !x.GetType().Name.Exists());
The expression passed to the Delete method doesn't make much sense in other contexts, we just use it to supply a filter that wil match anything.
However, instead of deleting everything and reindexing all your data, I'd recommend doing the following:
I have a custom indexing scheduled job. The files that needs to be indexed varies based on few dynamic criterias. Hence I want my schedule job to clear all indexed data and start indexing afresh. So is there a way that I can clear the index programatically?