Here is my opinion on how to kill the king

1. Kill mobzilla (optional because the second part)

2. Defeat the King dungeon (you can use the ultimate armor if you can't kill mobzilla)

3. Get a diamond or two

4. Go to the red ant dimension (only if you have the spawn egg)

5. Give the diamond block to the prince (recommend the young one because the adult is too big)

6. Spawn in the king

7. Mount on your young prince and attack the king

8. You kill him

Yes this is how i beat the king in my survival today in like 10 minutes (multiple of them aswell)