Secret cat companion:
First, reach England in the story. Then, travel to the city of Norwich, east of the starting area, in the East Anglia region. There is a hidden side quest in Norwich that will unlock the cat for your ship. It does not appear on your map or compass. On one of the roads in the town, you will find a white cat standing near a red norse gate, down the steps from the Dane's house. Talk to the white cat. After the cat runs away, talk to the child, and follow him —- the cat will run away when the kid gets close. You need to interact with the cat to grab it before it escapes. If it does escape, follow the child to the next location to try again. Capture the cat to complete the "Freyja's Friend" side quest. The cat will now appear on your longship.
The following five skills are recommended to unlock as quickly as possible in Assassin's Creed: Valhalla. Note: You can respec you skills at any time.
Auto-Loot: You do not have to keep press a button to collect loot from bodies. You will automatically collect everything when you walk over bodies.
Breakfall: This automatically causes your character to roll when jumping from a high place, making fall damage negligible.
Counter Roll: This is the only way to counter Rune Attacks. Counter Roll allows you to vault over an enemy preparing a Rune Attack and strike them from behind.
Dual Heavy Weapons: This allows you to easily wield two heavy weapons at the same time. Your heavy attacks are much quicker, and your special attacks can unleash tornados of destruction, easily wiping out entire groups of enemies.
Missile Reverse: This allows you to catch and throw back arrows, making it possible to deal with archers quickly in combat.
Easy "Equine Attack" achievement:
The easiest method is to do this on the straw puppets (practice targets) at the Hidden Ones Bureau, at your settlement in England. To perform an assassination from a horse, you must tap [Crouch] while on the horse (this will make you stand on the horse). Then, press the button prompt that appears to perform the assassination. If you do not see the button prompt appear, try damaging the straw puppets a bit by kicking them from your horse. When they are low on "health," the button prompt should appear. Since these practice targets do not move around, it is easier than doing it on real enemies.
Easy "Flying Eivor" achievement:
An easy area to do this is in the starting region of England (Ledecestrescire), in a place called Venonis (northwest of your settlement). There is a Roman artifact collectible (white dot/mask icon on the map). Where this artifact is located, there is a "Goliath" enemy on top of a large pillar. You can see him by using Odin's Sight. Use the ladder to climb up the pillar, shoot the red lock on the second ladder (use Odin's Sight again to highlight it), and reach the top. Then, allow the Goliath to perform his red rune attack where he grabs and throws you. Sometimes he will try to rush towards you and accidentally fall down to his death. It is recommended to create a manual save before trying this so you can just reload and retry immediately if it does not work correctly.
Easy "Not The Norse You're Looking For" achievement:
An easy place for this is in the "Essexe" region (southeast part of England), in the town "Colcestre". Travel to the viewpoint in Colcestre. From the viewpoint, look west, towards the center of town. There is a pond in the center of town with some guards and monks walking in circles around it. Simply go to the monks and press Triangle to blend in. Then, walk alongside them and wait to pass by some guards. You will get the achievement when you pass by a guard so closely that the assassination prompt appears. It will only take a few seconds in this location to get it. Note: This only works in "Distrust Areas" (areas that have an orange indicator at the top of the HUD). This town is a Distrust area.
Easy "Overdesign II" achievement:
A "hard difficulty" soldier is one that is several levels above you and has a red skull left of their health bar. This does not refer to the in-game difficulty. You can do this on the Easy difficulty, which is recommended because those enemies are otherwise tough. Go to the southernmost viewpoint in Hamtunscire (Level 340 region), in the town Werham. There are many "Standard Bearer" enemy types in this area. They are large enemies with spears and yellow names. Make sure the red skull appears next to their health bars. You should be under Level 300. If you are already above level 300, simply reset your level in the skills menu. Also, set the game on the Easy difficulty for Combat so they do not kill you as quickly. Before you do anything, create a manual save so you can quickly retry if things go wrong. Two of the Standard Bearers are at the northern gate entrance of the Werham town. More are walking around at the town square. Whittle down their health so they will die within one hit of your bow (Light Bow is recommended since it has more arrows and shoots quicker). Next, set yourself on fire. Either throw a torch at the straw around town, or at one of the straw roofs; or wait for the Standard Bearers to throw fire bombs at you. Wait until the red "fire icon" in the bottom left corner of your screen has completely filled up. Then, either shoot a Bearer with arrows while you stand in fire, or quickly run up to them and melee-kill them from behind before your fire runs out. However, arrows are the easiest option. Repeat this until you have killed three Bearers while on fire. You do not need to kill three in one go while on fire. It is fine to kill one at a time, allow the fire go out, set yourself on fire again, and kill the next one. You just cannot exit combat. If you exit combat, it will reset the count and you must kill three Bearers again. Note: There are also other shielded enemies in this area that wield Morningstars -- killing other Morningstar enemies also counts.
Easy "Row Rage" achievement:
An easy location to do this is in Lincolnshire (England), in the river south of Botolphston. It usually spawns 5+ boats close together. All you need to do is call your longship (from the item wheel, select the longship while near water), then open the sails and ram into five small fisher boats to get the "Row Rage" achievement.
Easy "Silent Viking" achievement:
For the "Silent Viking" achievement, only melee takedowns count (bow stealth takedowns do not count). You need to defeat ten enemies in a row without breaking stealth/without being seen. A good place to do this is the town of Ledecestre, two viewpoints north of your settlement, in the Ledecestrescire region. Create a manual save before starting. Then, take out enemies one by one. You can take as much time as needed -- just do not break stealth. Other good locations are any large towns (look for enemies who stand alone by themselves) and some Raid locations (they always contain many enemies, but are a bit more difficult than towns). The "Chain Assassination" skill (yellow skill tree) is extremely useful with this. It will throw an axe at a second nearby enemy to stealth kill them. It is acceptable to use this skill, as it counts towards the ten stealth takedowns -- just make sure you are not seen while using it. This is perfect for when two enemies are walking in a line, as you can take out the one behind and throw an axe at the one in front. It only works when the second enemy is a few meters away, not when they stand side by side.
Accomplish the indicated achievement to get the corresponding number of Gamerscore points:
Rampage (10 points): Complete your first raid in England.
It's Alive! (10 points): Create a Jomsviking.
Tranquility (10 points): Complete a Standing Stone puzzle.
Equilibrium (10 points): Complete 3 Cairn challenges.
Old School Treasure Hunt (20 points): Collect 5 Treasure Hoard rewards.
Skadi's Hobby (10 points): Perform a 150m slide in the snow.
Witch Hunter (20 points): Defeat one Daughter of Lerion.
Equine Attack (10 points): Assassinate an enemy from your horse.
Silent Viking (10 points): Assassinate 10 enemies in a row without triggering a conflict.
Everyday Life (10 points): Complete 10 World Events.
Overdesign II (30 points): While on fire during a fight, kill 3 hard difficulty soldiers without breaking their shields.
Completionist All the Way! (30 points): Complete all territories.
Row Rage (20 points): Ram and destroy 5 boats in under 2 minutes with your longship.
We're in the End Game Now (30 points): Reach Power 280.
Builder (20 points): Reach settlement level 3.
Home Sweet Home (30 points): Reach settlement level 6.
Home Decor (20 points): Place an item on each settlement cosmetic spot.
Not the Norse You're Looking For (20 points): Successfully pass close to a guard in a distrust area by blending with a group of monks.
Face My Might! (20 points): Equip 8 abilities.
Ultimate Refinement (30 points): Fully upgrade and enhance a piece of gear.
Flying Eivor (20 points): Get thrown 30 meters away by a destroyer or a housecarl.
Twinkle Twinkle (10 points): Release a firefly in your settlement.
Archaeologist (20 points): Complete all Roman collector challenges.
Orlog Champion (20 points): Beat all the Orlog players.
Good Catch! (20 points): Catch a fish of each type using the fishing line.
Full Mastery (30 points): Spend your first Mastery point.
Is There Anybody Out There? (10 points): Light the unlit braziers on Hadrian's Wall.
Slam Master (30 points): Complete all the flytings.
Master Hunter (30 points): Defeat all Alpha animals.
Dreamcatcher (20 points): Destroy 10 Curse Symbols.
Additionally, there are 20 secret achievements:
The Saga Begins (10 points): Complete the Prologue.
To England! (10 points): Leave Norway.
Hard Choices (10 points): Complete the Grantebridgescire Arc.
The Order Is Revealed (10 points): Complete the Lunden Arc.
The Good Saxon (10 points): Complete the East Anglia Arc.
Take My Hand (10 points): Complete the Cent Arc.
Calling in a Favor (20 points): Complete the Suthsexe Arc.
The Enemy of My Enemy (20 points): Complete the Wincestre Arc.
In the Footsteps of the Gods (30 points): Complete the Asgard and Jotunheim quests.
As It Was Foretold (30 points): Complete "The Prophecy" storyline.
England Subdued (30 points): Complete Hamtunscire Arc.
Disorder of the Ancients (50 points): Eliminate all targets of the Order of Ancients.
A Picture of Grace (20 points): Run through 30 breakable objects.
Pioneer (20 points): Reach Vinland.
Caladfwlch (30 points): Draw Excalibur from the Stone.
It's Not a Bug, It's a Feature! (10 points): Complete your first Animus Anomaly.
Seahorse (10 points): Swim a total of 3km with the horse.
The Hidden Truth (30 points): Obtain all video fragments and watch the hidden truth video.
Godly Reward (30 points): Obtain Thor's Helmet.
Worthy (30 points): Wield Mjolnir.