with my 720mm arty loki, i was able to harass groups of frigs by targeting the weakest and most expensive-looking one that’s stationary (or has minimal transversal velocity), decloak and fire a single volley, then recloak quickly. most of the time the target should die and his mates would be too slow to react.
groups of bigger ships (like cruisers), i usually avoid, coz they’ll survive the first volley.