>>281661793
The story doesn't flesh out Gudao/Gudako's power very well until the Lostbelts, so in the beginning it is like "how exactly are you going to beat Goetia without any servants?", but in the Lostbelt arc it is explained that they can summon temporary shadow servants to help in battle, and the closer they are, the more they can summon. So Gudao/Gudako needs to be close to summon efficiently. They can do summons up to a range of, iirc, 2km, but that limits them to only 2 shadow servants, and it puts a huge stress on their body to maintain it at such a distance.