His method of using brute force worked because Entropy always followed Time Traveler into the past. So by reversing time repeatedly, he kept taking on more burden of Entropy until he reached his limit.
That's also why he believed brute force could work against Ragnarok. But he couldn't argue against Wandering Thief because he clearly hadn't completely defeated the three brothers.
After so many years of effort and death, the three cosmic brothers were still alive. They may not be doing well, but they could still return to challenge him.
He was proud and confident in his strength, but he had to admit that a method fell short, even if only slightly, which is why he decided to use "sophisticated" methods to take Ragnarok down. So he kept his mouth shut now despite his immense displeasure.
