Flying Hornets
As William only had one tail form, he kept using his fire element, alternating it with lightning as well.
Fire was domineering, but lightning had a strong purifying effect against fog and monsters. Both became his sharp sword against the monsters and fog, killing his path deeper without any hindrance.
During all this, he kept feeling more heat and pain from his chest. The seal seemed to get more stirred up the deeper he marched in the fog. As he guessed, the fog at the outer zone was fainter and thinner than what was in the deeper areas.
He used the place created by his grenades to rest and keep weakening that seal. He used arrays, trying to crack another chain in this seal and enable him to use two tails at least.
Yet after all these hours he didn't succeed, not until now.
"Phew! I finally broke another part of this damn seal," he stood up, taking the form of two tails, enabling him to use two elements at the same time.