• gundred years' war,
• bundred years' war,
• nundred years' war,
• jundred years' war,
• uundred years' war,
• yundred years' war,
• hyndred years' war,
• hhndred years' war,
• hjndred years' war,
• hindred years' war,
• h8ndred years' war,
• h7ndred years' war,
• hubdred years' war,
• humdred years' war,
• hujdred years' war,
• huhdred years' war,
• hunsred years' war,
• hunxred years' war,
• huncred years' war,
• hunfred years' war

The adjective form of "Hundred Years' War" is "Hundred Years' War". Adjectives do not change form based on whether they are used attributively or predicatively.