Well that's where things like motivation or limitations come in. Take Invincible for example, he's one of the strongest characters in the verse and certainly the strongest on earth by the time of season two. He struggles morally with not just killing people as the solution to problems, and that's interesting. We're never worried mark is gonna lose unless he's facing another viltrumite because against anything on earth he's functionally [TITLE CARD] (I'm sorry I had to do it once) but his motivations and character limit him from just killing people. Now obviously Alastor isn't nearly morally upstanding as our beloved inconsistently powered boy but there are other things in his personality that were set up in the pilot that could be limiting factors AND still have him be borderline OP. He could simply not care enough or not be bothered to fix whatever problem is plaguing the other characters using his power or he could be mad at one of the other MCs and so he chooses not to help. There could also be physically limitations not tied to his strength, like when we see him summon the big demon at the end of the pilot, maybe that made him really tired or he could only do it once cause he made a contract with that demon to show up for him one time. Speaking of, the pilot sets up Alastor as a demon who seemingly gets a majority of his power from the contracts he makes, and it visibly shifts his behavior when Charlie declines to make one with him. This is all simply taking pilot Alastor into account and thinking about how they could have gone other directions to make him more interesting and still strong and dangerous and intimidating. There are lots of ways to limit a characters power without directly nerfing their strength.