User blog comment:SoulRise99/Chapter 526: August is the REAL child of Zeref and Mavis; as well as the WEAKEST Mage in Alvarez!?/@comment-47.150.126.254-20170318200437/@comment-7418318-20170318201105

Not to mention that Lucy couldn't summon Spirits against him.

While summoning the Spirits is holder magic, the Spirits themselves are sort of caster mages, so in all likelyhood August would be able to copy the magic of the Spirits that Lucy summons.

Which means she could only really use Star Dresses. And while those are awesome in their own right, it'd still reduce Lucy's effective fighting power to one third of her potential or even one quarter if she gained enough magic power to summon three Spirits and a Star Dress for prolonged combat rather than a last ditch gamble.