The thing with Nimrod is he is a very Doomsdays-ish character. He can be beaten if he encounters a situation that he hasn't encountered before or that he can't anticipate such as Nightcrawler teleporting his arm off, Rogue absorbing several powers simultaneously, Sebastian Shaw crashing into him at terminal velocity, etc. Rogue and Nightcrawler tried the same ploy during the next encounter. Rogue got her uniform shredded and nearly suffocated Colossus and Nightcrawler was forcefully and traumatically bampfed across town so bad that his powers misfired while fighting the Marauders the next night and he was put in a coma by Riptide and Vertigo and left the X-Men.
The problem is he adapts and it's very hard if not impossible to beat him the same way twice. I say beat and not kill because Nimrod can be reduced to a scrap pile and he'll just teleport away and repair himself while devising some new strategy. The only way to kill him is to take him apart molecule by molecule, vaporize him or annihilate him. In the way he repairs himself, teleports, and is able to create counters to every power he's faced including magic points to the fact that he can convert matter to energy and vice versa and that he can generate exotic particles. Nimrod's other weakness is that he adapts to the point where he started becoming more an more human and grew a conscience up until he was taken over by Mastermold and turned into an unstoppable monster.
In terms of the Juggernaut battle, he obviously fought and killed Juggernaut, a supposedly unstoppable opponent in his reality. I've never seen a more pathetic Juggs performance than being caught of guard, thrown across a street, severely beaten, and then paralyzed/about to be killed by some kind of magic-countering sonic frequency.
I said all that to say this. Mangog will fight Nimrod and he will win but he won't kill Nimrod. Nimrod will go away for a week or two and come back. He may be reduced to powder 9 times, but on the tenth, he'll figure out a way to kill Mangog.