This build doesn't give you the theoretical max dps, but it will give you something that is actually useable in-game. Swapping something out for [Primed Ammo Stock] is an option, but will come at the cost of major damage unless you have a riven that can replace 2 of the mods
Primary Merciless is there to shorten the reload a little to help with the small magazine, and extra damage is a nice bonus.

The [Phantasma] is best used against hoards of enemies, being able to spread status easily. It does struggle quite a bit against stronger single enemies (acolytes, bosses etc.) so I recommend bringing a secondary that pack a real punch, like the Laetum.

For rivens;
Swap out Critical Deceleration, but beware that you will burn through ammo if you dont have -fire rate.
+multishot, +status chance, +mag size and -fire rate are the ones to go for. Avoid +fire rate at all costs.

//RANT//
For some reason, all the builds for this weapon seems to force [Shotgun Barrage] on it, which is the last thing you want to do in my experience. With the magazine size being only 11 (15 with [Revenant]), and it being a status weapon, you should decrease the fire rate instead.
For those that don't know, status DoT (damage over time) occurs in 1s "ticks". In the Phantasmas case, that means your main source of damage will occur in these ticks. Thus increasing the fire rate will not only be obsolete in practice, but harmful, as you will chew through ammo while already dealing enough damage to kill most enemies.
//RANT//