Formation 3x2
Epidemius
Plague drones (5)
+ Death heads
+ Plague Proboscis
+ Standard Bearer with Banner of Swiftness

Offensively The reason this is very deadly is the poisoned attacks from Death heads, Plague Proboscis and Plague Swords because of Epidemius has the lesser Locus of Virulence making all poisoned attacks auto wounding on a hit roll of 5+ so it looks like it was intended for this.

Defensively Both plague drones and Epidemius are Monstrous Cavalry so Epidemius is more protected and as Plague drones are hovering and not flyers they can be joined by him they only follow the flyer rule of moving 10 wile epidemius moves 5 due to the banner of swiftness.

Feel free to comment if I miss anything