On Cataclysm, four people doing the same wrong thing is better than four people doing four different right things, a commander can help the team getting through the level a lot. It is not mandatory, but I strongly recommend at least one person can stand out and make the decisions. Don’t be shy to make the call to your teammates in voice chat.

Career: Mercenary, Foot Knight, Grail Knight, Iron Breaker (Non-Drake Gun), Slayer, Zealot, Warrior Priest of Sigmar

Job: Actively melee engage enemy. Create space for other non-front line teammates. Do the sacrifice move when necessary.

Analysis: A good front liner is the cornerstone of a successful team. You are the one who should never retreat first. Building a barrier between horde and your DPS and try to control horde/elites a fair amount of distance from them so they can maximize their damage. Ping and dodge the specials mixing in the horde as much as you can. Only use range weapon to kill specials when you know your teammates cannot help you. Pull the Monster aggro when your team is trying to kill it.

Career: Huntsmen, Ranger Veteran (Shotgun), Iron Breaker (Drake Gun), Engineer, Waystalker, Battle Wizard, Pyromancer, Necromancer

Job: Actively range engage enemy. Deal the highest amount of damage. Focus on your own position. Prioritize specials.

Analysis: Heavy range damage dealer. Should have the highest damage/special kills in the run. Always trying to get the best position to safely dealing the most damage. Only use your melee to block/kite/THP regen/kill single armoured elites. A good Ranger should have as little melee kill as possible.

Career: Ranger Veteran (Crossbow/handgun), Handmaiden, Shade, Sister of the Thorn, WHC, Bounty Hunter, Unchained

Job: Do those dirty jobs that no one wants but necessary. Sometimes depends on team composition, can be a Front Liner or Ranger.

Analysis: Most of time, you are the utility player of the team. You should have a clear overview of your team state. Clean the mobs that somehow get behind your Front Liner. Fully engage (melee or range depends on your career) if your Front Liner feels stress from extra elites. Kill the specials that your Ranger didn’t kill. Help your Ranger to reposition (killing the mobs that are harassing them etc) so they can focus on damage. Inform your teammates if enemies are coming from backline.
You can become a Front Liner or Ranger when necessary. E.g Handmaiden/Ranger Veteran can be Front Liner, Bounty Hunter/Unchained can be Ranger, but they won’t do the job as good as other ‘main career’.

On Cataclysm, a skaven/chaos horde is made up of three weaves. There are three kinds of weaves. All three weaves in the same horde are the same kind.

Shield wave:
Skaven horde: slave rat + shield clan rat. Chaos horde: fanatic + shield marauder.
Easiest one to deal with, but you will spend a lot of time if your front liner doesn’t know how to open shield or lack of Ranger.

Elites wave:
Skaven horde: slave rat + Storm Vermin. Chaos horde: fanatic + Mauler.
Horde with highest amount of hp. However using some career skills can shred them easily.

Berzerker wave:
Skaven horde: slave rat + Plague Monk. Chaos horde: fanatic + Berzerker.
Horde with highest amount of damage. Try to use some range attacks (even Front Liner/Roamer) to kill monk/zerker before they get close. High stagger career skills also help a lot.

The roaming ambient elites on the map are not dangerous. However when they got pulled by horde, they become very threatening. Try to kill them ASAP when you feel that the horde is coming.
Also the elites type can indicate the race of next horde. E.g If you find out most elites/roaming enemies are skaven, then next horde would be skaven horde.

On cata, you can have up to 5 specials and up to 3 per kind and ignoring threat level. Which means in the worst case you can have 3 assassins + 2 hook rat + horde + Monster.
There are many guides/videos available now to teach you how to deal with them individually. I will only mention priority/ important points on Cataclysm difficulty.

Priority high: Assassin, Hook rat, Gas rat
Assassin and hook rat are the most annoying control specials. Try to get some range breakpoints for them.

Gas rat is also very threating due to their super high damage and its nature of splitting the team. Once gas rat hp drop down to half, it will go suicide mission. If special sniper doesn’t have time to kill it completely now, at least deal enough damage to stop it from casting gas.

Priority medium: Leech, Ratling
Leech has very consistent move and distinct sound, making them the easiest one to dodge. However they are still a control special with a decent amount of hp, so focus them after other high threatening specials died.

Ratling has very high stagger resistance and huge damage potential in mid/close range. On Cataclysm most of range/melee attacks cannot stagger them out of their firing. But they can be baited fairly easy and kill horde/elites/other specials very quickly, to focus them after other high threatening specials died.

Priority low: Blightstormer, Firerat.
Blightstormer can be quite annoying since they tend to teleport far away and then cast the spell. But the storm damage is surprisingly low and will also destroy enemy horde/multiple elites. If you don’t want your team to split up because of storm, just let Front Liner jump in. Once the storm has a player, it will stop moving until the player gets out.

Firerat shares the same high stagger resistance of Ratling, but it only engages at very close range and its damage is fairly low. It also melts horde easily. Focus them when you have everything else done.

Due to high hp and different threat level, if you team don’t have high Monster DPS and don’t use potion or don’t focus on it, it is guaranteed that you will have horde + specials during the Monster fight. Don’t hastate about using resources on them.
When the horde come in during Monster fight, the one got aggro should pull the Monster away from team/horde and rest of team should go to intercept the horde. Don’t be too far away from the rest of your team though since you may will have trouble to kill specials. Only focus on the Monster when the horde is completely ended.

To learn how to efficiently kite Monster and their deep mechanic, use this guide.

To learn how to efficiently fight Beastman and their deep mechanic, use this guide on fatshark form.[forums.fatsharkgames.com]

Keypoints:

Ungor/ Ungor archer:
Dodge arrow. Be careful about their long spear poke.

Gor:
Mixing push during your attacks. Be careful about their fast head bump.

Bestigors:
Dodge charge, only block when there is no way out. Try to use range weapons like shotgun/handgun/conflag/hagbane to stagger them if multiple bestigors start charging. They have a cooldown of 10 sec on charge so once you stopped them they will melee engage.

Standard Bearers(A.K.A. Wargors):
Engage to destroy banner when it’s relatively safe. Also this job should be given to a Tank/invis career since they have a higher chance of survival. Once the banner is broken the Wargors can be left alone since they only carry one banner. Retreat to force Wargor to pick up banner when necessary.

Minotaurs:
Dodge its charge at last second. Don’t block too many attacks because of shield broken. Learn how to kite efficiently.

Beastmen horde:
Dodging charges, block/push mobs, dodge arrows as much as possible.

The whole Intervention system is quite obscure. If you want to fully learn this deep mechanic, use this reddit post:

Keypoints:

Threat Level

Basically means currently how many enemies are there in the game. The more enemies you have, the longer next horde will come. Different enemies will rise the Threat Level differently, with Monster rise the Threat Level most. If the Threat Level is high enough, the horde/special spawn will be delayed or even don’t spawn whatsoever.

Pacing Circle
The game spawns units in a circle with three phases. After Phase III, the circle goes back to Phase I.

• Phase I: Spawn specials, roaming enemy, maybe horde. If you got horde at Phase I, there will be no horde in Phase II.
  
• Phase II: Spawn specials, roaming enemy, maybe horde. If you didn't got horde at Phase I, the horde in Phase II is guaranteed.
  
• Phase III: Spawn only roaming enemy

How does this help? It means after killing a horde, there will be a period that nothing heavy will spawn. It is the best time to get supplies/push forward. But when specials began to spawn again, the horde will come very soon. Regroup when you hear the special spawn sound.
On Cataclysm the time interval between each Phase is very small. You will most likely be fighting Monster and horde + specials at the same time if you don’t manage your Team Pace well.

Rush/Speed run
Rush: Whenever you run ahead of your team, the game will spawn disabler near you.
Speed run: Whenever you or your team is pushing forward quickly without killing enough enemies, the system will spawn specials and elites nearby.

Trigger is a certain area in the map that will spawn Monster or Patrol. You active the Trigger by entering a certain place of the area or exit the area. One Trigger can only be activated once per game. One Trigger can be empty, a Patrol or a Monster. The nearby two Triggers can not have the same spawn type.
By carefully manage your pace, you can handle Monster fight more easily. Before entering a potential Monster Trigger, wait for the horde and kill it. By doing that, even if your team has low Monster damage, you will still have a period that you can damage Monster freely without worrying about horde/specials.

To learn all Patrol Triggers in the map, you can use this All Patrol Paths on All Maps by kuli
To learn all Monster Triggers in the map, use the mod Level Debug by Craven.

• Phase I: Start gathering your team when you hear the special spawning sound. Depends on how many roaming elites ahead, push toward next ideal horde fighting area or retreat to last ideal area.
  
• Phase II: Fighting together. Don’t push until all three weaves of horde are finished.
  
• Enter potential Monster Trigger after Phase II/during Phase III.
  
• Phase III: Pushing toward next ideal horde fighting area. Kill as many potential threaten roaming elites as possible. Can separate reasonably to get supplies.

While it is not recommended, a high skill group can partially ignore the Ideal pace mentioned above, since they can handle a more complicated situation at high difficulty.
A high coordinated team can pace faster since they can group/push/retreat together very quickly during Phase I.

Ideal horde fighting area:
Ideal horde fighting area is where horde cannot completely surround you. The best option is to find a place where horde only comes at you at 90° at the front. 180° is less ideal but acceptable. >180° is unacceptable, better start kitting them to a more desirable position.

For a real game analysis and showcase, I suggest watching j_sat Cata Coach series on his youtube channel.