Assassin's Creed Origins

This is the longest achievement in the DLC. Assuming you start the DLC at level 45, the previous level cap, you'll need to do quite a lot to get to level 55. Completing all of the main story quests will put you around level 50-51 or so. While completing story quests, I highly recommend completing all the locations that the story quests take you to. Many of them are extremely easy (read the stelae, loot the treasures, etc.) and still give you 750-900 XP just for those simple tasks.

There are thankfully a ton of side quests in this DLC to help you get to level 55. In particular, I highly recommend obtaining the side quests in each Afterlife location and completing them while you're there. Each Afterlife location has an overarching side quest with 3 sub quests. Each sub quest is a very simple fetch quest that gives you 5000-7000 XP, and then the overarching quest will give you 7000-10000 XP. These are always fast and easy, so take advantage of them when you travel to each of the four Afterlife maps during the main story quests.

To finally reach level 55, you should expect to complete all of the main story quests, a solid 10-15 side quests (remembering many are small and fast), and a bunch of locations along the way. Many of the forts are worth 1200-2500 XP for completing them too, making them worth it. Once you finally reach level 55, this achievement will unlock.