Best IPS is Slash. It's an exponential 1.3x damage effect.
Easy weapon to build is Cyngas. Without a riven, I'd go with all 4 60/60 mods + rubedo-lined barrel + dual rounds + automatic trigger + zodiac shred. This gets you a decent amount of damage that doesn't dilute the status effect pool(the combined elements don't have a proc in railjack), 100% status chance, and a bias towards slash procs.
This is how I have mine setup(can kill the highest level crewships in under 4s after I destroy their core for faster loot):
I also ran a Puncture Phaedra build for awhile; it