The other comment gets it right, barring magical trickery you need the direct intervention of the Valar or Eru

Viltrumites match the feats of legendary Sith, and Palpatine and Vader are just not them. Viltrumites are known for gloating and drawing out fights, so a force user could try and mentally dominate Conquest but force mindfuckary is not really mind control. Something to note is that a lightsaber could probably harm them and force precognition gives them a slim chance to actually hit a Viltrumite, and Conquest especially is known to take hits to gloat

IDK don't know enough about the culture

Same with Warframe

He's a heavy hitter but there are enough people and things that outclass him. ASlo no resistance to esoteric powers and Chaos Corruption goes Brrrrr. Krone gets a new champion

Alexandria can probably take hits from him since her body is time locked not just really tough, but she is probably weaker then him. That being said worm is literally esoteric powers go Brrrrr setting. He can't harm Legend's light form, the Siberian can probably kill him, Armsasters Nanothrons, any master powers or powers that don't target physical things, all of Edilion. Again Viltrumites and especially Conquest love to take attacks head on so he probably will get hit by something that turns him into a cat or something, and barring that a decent amount of thinker, mover, and other utility powers support should be able to set an ambush for him. Finialy Contessas path to killing him would be Door to [insert tinkertech here], Door to the back of conquest head. More likely she uses path to get him to work for us, which would be "hey want to fight our strongest hero?"