While the effort to replenish the losses which Zane had suffered in his battle with Gwen was minimal, the time it took was far more grueling. Simply travelling to Reno alone took roughly an hour of his time.
After arriving at their destination, Zane and the girls would fight eight hours a day, nonstop, before returning to Gravestone, where they would rest. Only to repeat the process again early the next morning.
They spent nearly half a month doing exactly this. Focusing on their own personal development while their scouts searched for another nearby Tier I Horde King to fight, considering Gwen was far more monstrous than they could have expected.
Despite being level 25, Gwen was way beyond Zane's capabilities, who was at the same level. This must be the difference between a level 25 "player" and a level 25 Boss. Regardless, the group would need to improve their levels and equipment if they wanted to face such a monstrous foe.