It was a plausible possibility. Percival's Skeletons usually held the short end of the stick when it came to battles with beasts and other Awakeners.
This was because Awakeners and beasts had the options of raw force and magical offense. Percival's Skeletons—being only raw bones revitalized by soul energy—did not have the option of magical offense, unless of course, Percival equips them with a special armor.
Because of this, they suffered damage at the hands of beasts and Awakeners. However, these Demons, the Manor Vampires at least; they only caused soul damage, draining the blood and hence, the lifeforce.
But Percival's Undead Summons had no blood, and with no blood to drain, their lifeforce also couldn't be affected. This meant Percival, and his Necromancer Class was the necessary weapon to take down this Demon Gate, or any that would appear in the world.
