Both Khan and Liiza had refrained from expanding their existence and control over the environment too far in the previous battles.
For once, there was no need to summon natural catastrophes against mere mana beasts. It would have been a waste of physical and mental energy that the couple might have needed upon reaching their destination.
Moreover, Khan was still learning to use his broader version of his element, and the endless tide of mana beasts worked as perfect training dummies. Vanquishing them all with a single spell would have prevented any meaningful exercise, so the couple held back from doing it.
Nevertheless, there were external reasons, too.
The planet's mana was oddly resistant to external influences, seemingly actively working to retain its unpolluted state. It abided by deeper, instinctive directives, following a master it didn't want to abandon.
