Du You was right, the added ability of energy burning was indeed an essential skill.
Energy burning was not an active attack type, but rather a passive trigger. When an attack hit an enemy, it could ignite a portion of the enemy's energy, causing internal secondary damage.
On one hand, it could reduce some of the energy inside the enemy, and on the other hand, it could cause damage and immense pain to the enemy. Such internal damage was not easily immune.
Unless the energy itself was very advanced, exceptionally condensed, or a special type of energy, there was no way to be immune to this move. This move was especially useful against powerful and difficult-to-kill enemies.
After this evolution was complete, Du You did not continue to clear out the Giants but stayed within his territory. That's because Du You found that his territory experience was already very close to tier five, just a bit short.