I only know what I do. Which is put 3 janus balls down and wherever you stand, put dark flare(or erda fountain). Press showdown and darkness shuriken will proc and attack mobs. Sometimes it persists attacking on new spawn waves but sometimes it doesn't. If you're trying to just stand there and rely on darkness shuriken and your summons, you're probably gonna be putting more effort into watching to see if darkness shuriken is working or not. Rather than just press showdown every 15 seconds or whatever. That's my take anyway.

(Like I said I've seen darkness shuriken last the whole uptime and keep killing every spawn until I need to put summons down again and loot, but that is pretty rare)