Whenever there's 3+ mobs and you're out of MP, use Total Eclipse.

In terms of pure potency, it's better to use Holy Spirit than Total Eclipse unless there's 4 or more targets (5 or more if buffed by Requiescat), but ofc, you only get to use it a couple of times because you need the MP for Flash, and you'll also need MP for the next pull to keep hate. You'll have to make that resource management decision yourself.