The Apocalypse is the best weapon with attack of 597 and boosts HP up to 403. In order to get the Apocalypse greatsword, you need to complete the full story campaign.

After that you need to get the Dungeon Seal Key by completing the specific dungeons. Once you have got the key, you need to take on the quest called A Menace Sleeps in Balouve.

After taking the quest, go to the Balouve Mines dungeon. In the dungeon, you will come across the boss whom you need to defeat.

The enemies here are level 75 so if you are lower in level make sure to use some legendary weapons. Near to that place you will find a small gap in the wall, in which you will find the secret dungeon door.

Enter inside and keep going until you come across Apocalypse. You will find it on the floor at the first camp site which is found 5 floors down to the place you are right now.