In Monster Hunter: World, players can fight a lot of monsters that they've previously battled against in the game's main quest. Two monsters, however, are a lot harder to farm parts from than others.
Zorah Magdaros, the final boss in the low-rank quests, and Xeno'jiiva, the final high-rank monster players can face off against, stop becoming available as quests after players complete the storyline. This can make it difficult for players to farm the resources these monsters provide for precious weapons, armor, and other useful items.
But it's possible to face these monsters again in certain circumstances.
After defeating Zorah Magdoras, players will sometimes get an urgent quest from the guild once they've completed a random amount of hunts. This new quest gives players the option to fight Zorah Magdoras one last time. Be sure to quickly take the quest, though—it'll appear on your option quest list for a limited time only.
Xeno'jiiva is similar to Zorah Magdoras, because players have to grind a few hunts first to get these missions to appear. But there's a quick way to get these optional quests to show up. All you need to do is go into easy quests, load up the map, and abandon or complete them. After a few cycles, the quests should appear.
Another way to join these quests is to help hunters in need. Hunters sometimes fire SOS requests while in missions, asking for players to support them in battle. You can search for Xeno'jiiva and Zorah Magdaros related SOS signals from the join quest option. Be sure to act fast, however, because you aren't the only hunter who will be trying to get these quests.