Shotgun Elementalist buffs the slash bleeds, but puts it in weird 3 magazine size. Riven or magazine mods preferably should boost it up to 4. This way you can double click and burst an enemy with slash procs. (You can try to slot in Primed Ammo Stock for triple click burst)
Last slot is flex depending on your riven roll. Punch-through is great for mobs but you can use something else if you roll it on your riven.

Honestly what this gun needs is a higher base status chance to compete against other shotguns at the moment for end-game content. Maybe incarnon update or exclusive mod.