The only consequence that the person they were attacking created on Legion was to increase the consumption of spiritual energy needed for Legion to remain hidden. Aside from this, the person might as well be dead.
Legion could maintain this level of spiritual energy consumption easily, so they focused all of their might on the neighbor. The fight was made even easier because the neighbor that they picked was also being attacked by someone else.
So it was a two-on-one fight. This was a special situation because both their partner and the single enemy that they were ganging up on didn't notice them at all.
Even when the Mind Plague collapsed under the combined mind of Legion and their helper, the owner of the Mind Plague still didn't notice them. The owner of the Mind Plague was only surprised that they had been defeated.
After the victory came pursuit. Both Legion and their helper traced the defeated mind plague back to its source and pursued it to attack it.
