Zach's plan to kill Great Ugor by hitching a ride to the Underworld inside the Basilisk was one thing. There was just something they had to do before that. They had to get rid of the Basilisk.
The big snake would be the biggest immediate threat upon their arrival, and that was if they could hold out long enough inside its highly corrosive stomach acid. To improve their odds as much as possible, Zach needed to be as strong as possible. He couldn't afford to maintain his barrier forever.
The best would be if they could find a safe spot inside the Basilisk and relax there until they arrived. That wasn't likely. That meant they had to neutralize the Basilisk's stomach acid, then it, and then leave the corpse as soon as possible.
