For the sake of the plot I'd assume Mel won't ever use his powers like this. If he really wanted though Mel should be able to defeat any enemy he faces through the use of Revenge Counter. I haven't seen any limits to how often he can use the ability , so spamming could be a thing. We've seen how durable his body is; even against opponents who are far stronger than he was at the time he could still tank hits that would obliterate anyone else. So even if Zeldris is stronger, the revenge Counter should be able to take him out. We see prof of that through his reaction to the Revenge Counter he has already seen. The only counter point being how much damage would have to be taken to take down an opponent like the Demon King or Supreme deity.

Edit: I totally forgot about him facing an opponent who is physically stronger. Against an opponent who relies on magic to defeat an enemy he should win.