Assassin's Creed Valhalla

Story-related; cannot be missed.

This achievement unlocks upon beating Hel at the end of this DLC. The DLC consists of four regions, each with a specific boss at the end, and you'll need to beat them all in a single run to complete the DLC and unlock this achievement.

As noted in the Roadmap, like all roguelite games, the difficulty is highest at the beginning of your time with this content, and only gets easier as you play more. So before you fret about trying to beat all four areas, and getting frustrated that it's tougher than the base game or other content, you'll definitely want to put some time in to expand your skill tree by spending Memories, and get some new/upgraded Outfits to help. The only real randomized part of this DLC are what the rewards are for each encounter in each region. Aside from that, most things stay the same run after run, specifically the layout of the regions (your map actually stays uncovered permanently), the boss fights and attack patterns, enemy and encounter locations, etc. As a result, it's possible to provide some tips and guidance that will work in all runs, since there is actually minimal variability.

First off, and most important, is the existence of an exploit you can use to save you if you really mess up a bad run. The game autosaves right after you complete every encounter in this DLC, as well as right before every boss fight. What this means is that you can quit out and reload your save whenever you feel like you really screwed up, took too much damage, or even died. For example, you're having a great run and then those druid enemies in Nidheim really destroy your health with their fire attacks after you screwed up some assassinations. Rather than being stuck with the results of that fight (such as all the health you lost), you can pause and quit to the main menu, then continue your game. You'll be back to just before you started that encounter, allowing you to reattempt it again, hopefully with better results. You can use this during any encounter or boss fight to ensure things go better for you, or just that you succeed and don't die.

With that important exploit out of the way, here are some general tips and guidance that will help you be successful:
 

• Your early runs should be focused entirely on collecting as many Memories as possible. Memories are used to expand your skill tree, which is the most critical thing to do to make this DLC easier.
• Speaking of skill tree, while you can obviously cater your unlocks to your playstyle, you will unavoidably be in conflict during many encounters and all boss fights. I therefore highly, highly recommend that you buy skills "up" and off to the left. Going straight up and also left gets you health upgrades (very important), melee damage upgrades (extremely important), and melee resistance upgrades (critical). Also at the top of the skill tree is, in my opinion, the most important skill in the game: the skill that slows down time when you dodge just before an enemy hits you. This skill should be your top priority, so make a beeline up to it right away. Then, work on all of the health upgrades, melee damage upgrades, and melee resistance upgrades, with next priority being the critical hit and damage upgrades and ranged resistance upgrades. These will all make marked improvements in your survivability.
• Regarding the encounters and the available rewards, this will again depend a bit on playstyle (for example, I never use abilities, so I never prioritized encounters with abilities or ability upgrades), but my recommendation is that your top priorities should be health upgrade and weapon upgrades. I found that getting my melee weapon fully upgraded was a game changer. Enemies and bosses died so much quicker. Health upgrades are equally important since they drastically improve survivability. Next priority after those should be rune rewards, since you can get some really good runes during your run. Those prioritizations will only apply to Kaldstad and Nidheim though, since those two regions are the only ones where you need to make a choice and only get some encounters. In Dokkerland, on the other hand, you're free to go after every encounter since there are no branching paths. If you're confident in your abilities (and using the save exploit mentioned above as a backup), you should absolutely clear each encounter. For the ability and ability upgrade ones, since I didn't use abilities at all, there's no point in picking abilities just to have them. Same with the encounters that reward a new weapon. For all three of these encounters, I would choose the coins as a reward instead. Coins are extremely useful in the shop. More on that below.
• One more thing on encounters: the encounters where the enemies are already spawned are far easier than ones where you need to invoke them from a sword. If the enemies are there, you can stealthily take them out and the risk of taking damage is very low. For invoked encounters, you'll be thrown into combat with multiple enemies all at once. These are risky in terms of taking damage and losing crucial health. However, with a bit of skills and upgrades, these encounters in Kaldstad and Dokkerland should pose little threat and can likely be tackled without much risk. Nidheim is another story entirely. I highly recommend avoiding any of the invoked encounters in Nidheim unless the reward is something absolutely necessary (like your final weapon upgrade, for example), because the group of druids are extremely powerful, and can very quickly and easily overwhelm you with fire attacks, draining your health. If you maximize encounters in Kaldstad and Dokkerland, and use coins to buy stuff from the merchant at the end of each region, you should be able to run past almost every encounter in Nidheim, only doing ones where enemies are already spawned and only if the reward is worth it.
• At the end of each of the first three areas, and halfway through the four area, is a merchant where you can spend coins. Your absolute top priority in these shops should be to buy a weapon upgrade to continue working towards a max level melee weapon. If you desperately need health, that should be next priority. After that, check through the runes and buy whichever ones apply to your build. Things might seem pretty expensive, but if you take my advice above with encounters and choose the coin reward often instead of accepting something you'll never use, you'll be swimming in coins (I was often entering the shop with 1000-2000 coins on hand).
• Make it a priority to finish at least the Kaldstad side quest (discussed in Freedom Fighter (40G)) before worrying about beating the DLC. This side quest rewards you with the ability to revive once per run with half of your health left. This is extremely helpful since it effectively gives you 50% more health during every run.
• Lastly is weapon selection. I personally went with something fast rather than something slow and strong, such as two handed weapons. Most bosses move around pretty rapidly, and if your attack is slow, you're going to end up missing very often. The faster your attacks, the more effective you'll be. In particular, the first boss jumps around a lot, and the second and fourth bosses also move pretty frequently. I personally had great success with a flail that increased speed with each successive hit. I was killing any enemy in a single combo, and really wrecking havoc on bosses with a fully upgraded flail. Use whichever weapon you prefer, but I highly recommend prioritizing attack speed to help with bosses. 

Those are all of the tips I can offer, as the rest will come down to skill. However, thanks to that save game exploit, you'll actually have the ability to attempt each encounter or boss fight over and over, getting needed practice to improve your tactics and learn when to dodge to avoid damage. Keep at it, constantly growing your skill tree, and you'll see the game become progressively easier and less intimidating. Once you finally succeed and beat Hel, you'll unlock this achievement.