As Leon and Nathan continued to pressure Mako and Erin, they purposefully didn't commit fully and didn't rush in; not that it would be easy considering Mako and Erin were also firing back different elemental projectiles to stop their advance.
The reason why they were doing s was to keep Mako and Erin busy and distracted while Bill did the main work.
When they had started firing projectiles at Mako after Nathan's howl, Leon had signaled to Bill with his hand signs to not engage them and instead circle behind them and steal the crystal instead without them even knowing and make a run for it back to their starting position.
This was the reason why Bill had stopped attacking Erin, it wasn't because he was out of energy to continue, but rather he wasn't there at all. He was tunneling underground around them so that he could steal the crystal.
