Nidhogg battling Huangdi while Thomas is battling Yandere Clock's mage, if my assessment is on the money:

Thomas has to do nothing (thus using no mana), Nidhogg gets free mana from the reanimated dead Yandere's mage uses.

Yay.

Fun times.

Therefore, as long as Nidhogg can even keep Huangdi busy, Thomas can unleash whatever spells he has charged against Huangdi's master, while his magic is being depleted in useless activity.