Zatch was definitely a powerhouse, if he didn't have enough power he wouldn't be able to defeat them.
Did you even read the prequel? There were numerous enemies that Zatch himself never beat because of power, but through strategy. The narrative and story itself even showed how outmatched he was in raw power compared to the others and how almost all his major victories were through strategy, not brute force or power.
ALMOST ALL OF HIS NOTABLE VICTORIES WERE NEVER THROUGH RAW POWER.
Like Robnos that faked a kidnapping of one of Kiyo's friends to fight him. His basic spell was much stronger than anything Zatch had, and Kiyo only won by using a lightning rod to turn his own power against him. This was the guy who blocked Zatch's most powerful attack with one hand.
Or how about Barry? Who completely demolished Kiyo and Zatch in their first encounter. And then later on when he comes back, he solos an enemy that Zatch and his group barely managed to defeat prior to that guy receiving a power up from Zeno?
Or how about the battle against Pamoon who overpowered Zatch and only had a change of heart because of seeing how strong and resolute his character and determination was.
Or how about Demolt? A guy that required the entire team to defeat and a lot of strategy created by Kiyo?
Or how about Coral Q? Required Kiyo to set up an incredibly long strategy just to defeat him?
Or how about Earth? Who was revealed that he was holding back the whole time against Zatch? And even he needed help from Ted just to survive?
Or how about Rein? A guy who effortlessly destroyed Riou? Riou who prior to this, was completely dominating Zatch and Kiyo?
And so many more examples.
Zatch didn't make it so far because of raw power. He won by teamwork and strategy in almost everyone of his major battles. How did you even come to conclusion that he wins by raw power?
Even his most powerful spells require stacking Zaguruzemu multiple times before he could match the opponent's most powerful spells.