If you don't get this on your first playthrough, best odds are playing a level where 2 Seraphs spawn on the lowest difficulty (the final encounter on Nihil will have one) using your Vulcan's Ultimate to pull together the Seraphs and using the Persephone (Shotgun) ultimate to kill both.
Alternatively, 2 Seraphs will spawn on the first encounter on Nihil on the highest difficulty.