I did not write this solution it was taken from a reddit thread on Vermintide 2 for steam.
But this helped a lot when we were going for these achievements, so I figured it could help others.
MASTER OF CONVERGENCE
Legend: Line up all 3 Athel Yenlui leylines within 30 seconds.
This one was one of the easiest challenges. First, you want to clear out the arena. New creatures will not spawn until the event has started, so use this time to assign positions and make sure everyone is clear on the instructions. We had Kerillian, Kruber and Saltz each at a dial, with Sienna overlooking them all offering cover fire. Give a count down ("3, 2, 1, Go") and begin turning the dials at the same time. Once completed, you will see a prompt in chat. From here, finish the level.
Legend: Rescue all prisoners within 60 seconds on Against the Grain
For this you will want to bring a Handmaiden with the Decantor trait on her charm. She will also need a Potion of Concentration, and a buddy. Start the timer by unlocking the first cage. Once this happens, the wall will break down and someone will race to the first set of cages, 4 on the bottom level and 2 on the top. Handmaiden will remain at the broken wall. Once the first 4 cages are unlocked, a ladder where Handmaiden is waiting will fall. This is when she will use her Potion of Concentration, climb the ladder and unlock the 2 cages immediately in front of her. She will move forward to the ladder leading to the upstairs of the 2nd barn. From here, dash across the landing to the 2 cages on the top level. After opening these, drop down and open the last two cages. You will see a prompt in chat indicating you have completed the challenge.
Legend: Have the statue of Sigmar kill 3 Chaos Warriors on Righteous Stand
Where to find 3 Chaos Warriors:
RNG is the true test with this challenge - when and where you will find the 3 Chaos Warriors you need is not guaranteed. I ran this multiple times and there did seem to be a pattern in where they spawned, though. My strategy was to start kiting any Chaos Warriors I found once I dropped down from the platform of the building where you find the last Grimoire. If there were none there, that meant they would be in the meadow after the drop off (by the last Tome.) There were TWO instances where there were NO individual Chaos Warriors in either of those 2 places: (1) There was a Chaos Patrol in those areas, or (2) There was a Skaven Stormvermin patrol in those areas. Patrols earlier in the instance did not seem to effect Chaos Warrior spawns later in the instance.
Every run I have done of Righteous Stand has always had 1 Chaos Warrior spawn at the Church entrance, just up the stairs from the medkits/ammo box.
How to keep them alive until the blast:
This was the easy part- kite them. You can choose to bring a Battle Wizard for the block cost reduction, and some single target DPS (Shade w/ daggers) but it really does not matter. If you have trouble kiting you can make it a bit easier by using the stairs. Go up the stairs then drop down the side and go back around up the stairs. The Chaos Warriors will follow you, but won't be able to catch up to you. Make sure you have someone on Specials duty - an unchecked Hookrat can make this simple challenge a nightmare. If anyone in your party dies they will respawn inside the Church at the entrance.
NOTE: THIS IS THE ONLY CHALLENGE I DID NOT RECEIVE A CHAT PROMPT INDICATING I COMPLETED IT. DO NOT PANIC IF YOU DO NOT SEE ONE. Just finish the level, and check your book when you return to your keep to make sure it was completed.
YOU TAKE ME UP
Legend: Have 15 enemies be sucked up in one of Burblespue Halescourge's tornados
A pretty quick challenge. Start the fight, with slow DPS on Bubblebutt. When adds begin spawning, block and stand sort of close to the middle. Next a tornado will spawn, just back out of it. Enough adds should have spawned and will be sucked into the tornado. If successful you will see a chat prompt confirming the challenge is completed, and can now kill Bubblebutt.
A LITTLE SOMETHING IN YOUR EYE
Legend: Kill 25 enemies by dropping the cannonball on their heads in Fort Brachsenbrucke
First you need to spawn the cannonball, so do the initial cannon mechanic as you would normally. Once the cannonball has spawned, retrieve it and head over to the tower where the ammo box is. Have at least 1 person on the ground level below you. This person will want to be a bit tanky, I opted to go Ironbreaker for this, as all enemies will aggro to them. Once a decent sized clump of enemies has formed on this player, drop the cannonball on them. Someone will then take the cannonball back up to the tower, and repeat. It took me about 5 drops to get all 25 enemies. As usual, a chat prompt showed when the challenge was completed.
BLESSED BY THE FATHER
Legend: Have on player get "bathed" by all Nurgle buboes in the end event of Festering Ground
Another challenge where you want someone on the tanky side. Ironbreaker with Natural Bond is perfect for this. The person who is going to be "bathed" will want to stand directly under/in front of the buboes as they shoot them, everyone else in the party will be killing enemies and protecting the bather. The last batch of buboes had 2 that are in cubby sort of spots, you can just climb into them to get sprayed.
NOTE: I received the chat prompt that the challenge was completed prior the last buboe being broken, though I am not certain if that is a normal occurence or what the actual number of buboes is required.
Legend: Destroy the Screaming Bell and escape in less than 75 seconds after starting the event.
I tried this one 2 seperate times, and it appears to be bugged. The first time, we broke all the chains and we ran past the Rat Ogre. We made it to the Bridge of Shadows within 75 seconds, however the challenge was not completed. The second attempt, same strategy. Challenge was completed as soon as we dropped into the hole the boss comes from.
The timer starts as soon as the first chains breaks.
You do not have to be alive to receive credit. Handmaidens, get ready to carry your friends.
FEED ME, SINNER!
Legend: Have all four party members stand inside the ritual pool for 120 seconds in convocation of Decay.
Very simple, and not as strict as the despcription indicates. Stand a group on a pillar prior to jumping into the circle. Count down ("3, 2, 1 Go") and head immediately into the center of the pool. Stand back to back. The first time I attempted it, one of our party was downed, and it seems that made us fail the challenge. Do not jump, either, as that leads to a failure. My second attempt of this was done with 3 human players and 1 bot. The bot ran out of the pool, but it did not cause us to fail. A human player was hooked by a hookrat, but was released within the pool, and that did not trigger a failure either. The chat prompt indicating the challenge was completed appeared about 15 seconds prior to the completion of the ritual.
Legend: Ring the dinner bell and kill the Troll in less than 15 seconds.
I have tried knocking the Bile Troll off a couple times, but it takes longer than 15 seconds to get him in a position where he is easily knocked off. Instead, just take high DPS heroes (Pyro, Bounty Hunter, Huntsman, Shade) and kill him. 3 players can stand where he spawns, and 1 can stand by the dinner bell to start the event. I was able to do this without any bombs or potions, but if you're struggling to kill him in time then Shrapnel + Explosive bomb + Strength potion should help quite a bit.
NEW Alternative Method: 3 people go to the exit and wait, 1 rings bell and baits Troll to bridge. Bridge breaks, killing the Troll and the baiter.
Legend: Have Bodvarr charge into a Chaos Warrior 5 times
This challenge was the most random, and took the longest. The way I completed it was to damage Bodvarr halfheartedly until he was at about half HP. By then, enemies will have spawned with at least 1 Chaos Warrior. Kill everything except the Chaos Warrior, and be ready to block while Chaos Warriors swing at you for 10 minutes straight. We weren't able to figure out what made Bodvarr charge and a lot of the time he would just sit there doing his same knock-back slam, while repeating the same voice line over and over. We just continued killing everything that wasn't a Chaos Warrior, and letting the Chaos Warriors that spawned beat on us. Eventually the challenge was completed, indicated by the chat prompt, and we killed Bodvarr.
NOTE: We did try stacking, with the thought that if all of us and the Chaos Warriors were in 1 spot, then Bodvarr could choose any of us to charge and he'd hit all of the Chaos Warriors we had alive... But this seemed to deter him from charging even more.
MORE WHERE THEY CAME FROM.
Legend: Have Skarrik Spinemanglr kill 40 skaven.
Another challenge where you need a boss to hit an enemy, but faster. Damage the boss lightly, and after a time he will move into a corner and call in enemies to fight you. Kill the Stormvermin, as they will become an annoyance as more are added. Let the slave rats pile up, and Skarrik will kill them as he slashes and twirls around the arena. Once you see the chat prompt that the challenge was completed, kill the boss.
Legend: Complete Empire in Flames without using any of the event spawned explosive barrels
The hardest challenge out of all of them, but plenty do-able if you keep a cool head. There are 3 barrels that spawn in set locations throughout the map.
I am too lazy to take screenshots or make a video of their locations, I am sure someone else will soon if they haven't already. Here is a very brief description of where to find them.
At the start of the level when you first spawn in, turn around and look behind you. There is a shed, inside you will find the first barrel.
Behind the first building you have to walk through. You can jump down from the roof, or go through the doorway thats under the stairs inside.
On the roof of a building on the left side of the road between the cellar where the second tome is and the building with the second grim.
Grab a barrel, and progress through the map. Just make sure to be careful dropping your barrel in a spot where it won't get hit during hordes, and making sure to take out any Specials you hear before picking it back up. Don't go too slow, you want to keep a decent pace.
There are 2 walls I used to throw the barrels over so we did not need to carry them so far.
Before you drop into the cellar where the 2nd tome is, there is a building on your right. If you throw your barrel over it, it will land where you exit the cellar.
After picking up the 3rd barrel, there is a white wall to the right of where the 2nd grim is. Throw the barrels over it, and they will land outside of the house you jump out of.
DO NOT PUSH THE CART ALL THE WAY TO THE GATE PRIOR TO DEPOSITING THE BARRELS. If you do, you will not get credit for this challenge. Push the cart half way, then deposit the barrels, before pushing the cart the rest of the way.
DO NOT BLOW UP ANY OF THE EVENT SPAWNED BARRELS. If any of them explode, you will not get credit for this challenge.
RIDE TOGETHER, DIE TOGETHER.
Legend: Kill Deathrattler and then Rasknitt within 20 seconds
For this challenge you will want to dismount Rasknitt by damaging Deathrattlers controller on his back (I call it the baby backpack.) After Rasknitt hops down, damage him as much as you can. He will jump back onto Deathrattler multiple times, so try not to do too much damage to Deathrattler other than hitting his controller. Once Rasknitt goes into his kneeling animation, kill Deathrattler. As soon as Deathrattler is dead, finish off Rasknitt. The 20 second timer starts as soon as Deathrattler is dead, it does not matter how much time you spend prior to his death.