Let's keep in mind that Hedorah could only be successfully dehydrated by an electrical current running through his entire body, cooking all his tissues at once- and even then, if he was forcibly imobilized within it. And even then, he had an escape form that could outrun most Godzillas or anything else capable of bringing him back to the one specific spot where he could actually be surrounded and immersed with electricity- a form requiring the same treatment all over again, after which he had to be physically torn apart to prevent recovery and still had internal seeds that could (presumably) allow his regeneration if not specifically destroyed.
Just shooting him with a maser or heat-ray isn't going to do it; like a piece of meat cooked on the inside but still raw at the core, just heating his outside won't do it; hence why Godzilla's heat ray didn't have an effect. That cooks the outside, but it doesn't reach all the way in. So there's no reason to believe the spiral heat ray or burning Godzilla's heat would do the same- or even blasting him with an electrical attack. He needed to be specifically held immobile, twice, within a saturation chamber that could pump electricity through all of him at once... not an easy-to-achieve battlefield condition. And any Godzilla without the power of flight still wouldn't be able to kill him that way; he'd just escape and not fall into the same trap again. So I am not so convinced that Burning Godzilla could easily kill him. His defeat conditions require an awful lot- as long as he can move, or any attack is just hitting him externally, affecting his epidermis and diffusing rather than soaking into every fiber of his being, he's not going to go down easily (if at all).
Meanwhile, Destroyah, at proportionate scale, lost to Junior. His one trump-card, micro-oxygen injection, was overcome by Junior's regeneration. That doesn't bode well for the notion that only Burning Godzilla can defeat him- and I don't see any particular attacks (except maybe the laser katana) in the scaled-up version to indicate an increase in threat level to someone 'his own size.' Now, could the katana inflict fatal damage without the amplified regen of Burning Godzilla? That's the one argument I can possibly see in his favor.
And is it possible that the military could only take him down because Godzilla had already weakened him? Maybe. But Godzilla didn't even seem to have the ability to wear down or weaken Hedorah prior to the actual activation of the very specific, one-time trap gambit. So if we're measuring deadliness by ability to survive and continue dealing damage, I think there's a pretty strong case for saying Hedorah trumps in that department.
It’s part of my secret plan to create a fifth column in the US, pre-emoting our glorious conquest and the creation of the Canadian Empire, upon which the sun will consistently set after less than eight hours of daylight.
The grace of God is a greater gift than we can truly fathom; undeserved mercy is a kindness humbling in its sheer scope.
The Zone Fighter campaign is complete, with all episodes subtitled! PM me if you need a link location.