Meleoron is basiclaly an invinicible power. All you have to do is throw a can of the poison from the Rose while you're with Meleoron and you'd take out anybody. I guess you'd die in the process too but that's obviously still a good trade.
Meryem is pretty much completely imprevious to any normal tactics even in his first form. Even without powers like reading opponent's mind, he'd never let someone attempt a dangerous ability in front of him. For example if you hand him Shalunark's antennaes and told him to stick that into himself (no one else besides himself has the strength to stick that through his skin) it'd probably work, but he's not stupid.
Meruem is a quite beatable character, but not in a one on one or any fair fight. you need hax abilities and good planning to get him down, or you'd need many hunters with a strong oath such as gon's.
While i doubt meruem could have just went on to invade the world when the hunter association had enough time to develop a strategy, meruem in his last form who could read minds would even then be a hard opponent since he is incredibly smart.
he might have been able to conquer the whole world, but not 100%.
We don't know all the strong hunters yet, but i'd guess that with abilities like allukas or meleoroon's some smarty like pariston would find a way to stop him.
And i doubt those 2 are the only ones with suitable abilities.
BUt since netero would be unable to poison meruem when the altter could read minds already, it would be devastating, if they win or not.
As for the topic, there is noone in the genei ryodan who is able to hurt meruem except possibly feitan and phinks, and with some good ability, lucifer.
Meruem can one hit all of the genei ryodan as long as they dont use any ability like meleorons, and he can do that in a mere second.
So as long as lucifer does not have a possive ability like that dude who poisoned uvorgin he could imbue into his body, then erase the memory so meruem does not know it and then baut meruem to attack him, there is no chance that they could defeat the king.
On a sidenote, you could always use hunters who attack meruems psyche, possibly via komugi, or, as i said, i guess when it would be a lost situation anyway, i gess many hunters would decide to use a stong oath to kill the king.
The Royals are unaffected by the heat from the Rose explosion which melted rocks. Feitan's attack isn't anywhere close to that kind of tempreature (or it should have melted the surroundings). Phink's ability isn't even stronger than a Koh, because its restriction is lower than a Koh (he seems capable of still defending while charging up), and it's questionable whether Koh is enough to hurt a Royal let alone Meryem (Feitan's Koh did no damage to Zazan).
It really doesn't matter what combination of tactics you use. As long as you don't have an ability that allows you attack undetected (Meleoron, which is enough to defeat virtually anyone), you're going to die before you can even attempt to use your ability. The only way a human level character might have a chance is if Meryem agrees to not kill the guy ahead of time, because otherwise he can simply kill anyone before they have a chance to do anything. At any rate, ability like Meleoron's are extremely rare even in HXH probably because powers like his is basically unstoppable and it wouldn't make a very interesting story if you're allowed to attack someone while they have no possible way to detect you.