Every assassination list has a core of models that are required to get the assassination off. A good example is Siege. You rush up rangers for +2 to hit, move up, drop foxhole to get line of sight to the caster, feat, shoot twice (second shot coming from Reinholdt), eiryss shoots off any focus on your caster and then you plough all your other boostable guns into the caster as well, generally a Stormwall, a Defender marshalled or controlled by jr. The most important thing in most assassination lists is a DEF debuff or hit buff, knocking down the opposing caster is best but failing that you want something else that will make the caster easier to hit as that is generally the biggest obstacle in killing a caster. The other thing you need is threat range. You are't going to be able to walk over a beast/jack and headbut your opponents caster until you have fought through their entire army. They are going to be keeping their caster somewhere far away behind their models where they think their caster is safe. You need something that can reach out and touch a caster from a massive distance, the longer the better. Anything that allows you to move your opponents caster is tremendously valuable, particularly if you can move that caster without rolling (like for example Hellmouth). You can pull casters out of terrain, away from cover, out of auras (or move the defensive Aura) and it can make a very big difference.
Here is a circle assassination list from Warmachine Weekend (Craig Conroy).
Krueger the Stormlord
Druid Stoneward and Woldstalkers
- Stone Keeper
Eiryss, Mage Hunter of Ios
The important thing here is that Kruegar has telekinesis. Both Woldwardens can use geomancy to cast that spell for one fury and then Krugar can channel it through the gallows grove 3 times (or twice boosted), this lets you move things out of the way so you can get to the caster and move the caster up and turn the caster around so you gain +2 to hit. You also have Eiryss there to shoot off any focus or defensive buffs on the caster. The other thing this list has that is important is two units of shifting stones. One of which will be on the edge of advance deployment and the other around the Woldguardians. The Woldguardian can then be teleported to have the back edge of his base just within the stones and then teleported again by the advance deploy unit for 20" of teleportation shenanigans + 10" of telekinesis shenanigans. The woldguardian is a great assassin because as soon as it hits your caster is knocked down and the rest of the attacks hit automatically. You can also see that if this attempt fails you've given your opponent a free heavy beast and you don't have your other beasts in position to strike or go for scenario since they moved in order to cast telekinesis. There are two Woldgaurdians in the list but only one can participate in the assassination so if you are waiting till turn 2 or 3 to assassinate (which doesn't always happen) you can use the heavy as bait.
Playing an Assassination list
Know your threat ranges. That has to be the biggest key, know how close your opponent needs to be before you can kill him. As above you need a massive threat range and this normally puts the turn 1 assassination on the table. Ask your opponent haw far they are moving their caster. Know exactly how far they need to move to allow you to assassinate. Know the odds of the assassination, against what DEF or what ARM will your assassination start to become less likely? If you can't use one of your key pieces for some reason how does that change your odds. If you are playing the Siege assassination list and your opponent has blocked off your rangers this turn but is within the 19" you need to get the assassination off you need to know whether you can still make that assassination without the rangers or whether you need to throw away some models to allow you to reposition for the assassination next turn.
It's a different way of thinking about resources too, when you are playing assassination the only resources of your opponents that count are those that can stop you from successfully assassinating and the only resources of yours that count are those that you need to assassinate. The fact that you are thinking about the game this way can allow you to make very good trades that also look advantageous to your opponent. Generally you want their caster to spend resources, be on less focus or fury, you may also need to bait out key pieces like Mulg or a Bronzeback (protective fit and counter charge respectively). You can use things you don't need for the assassination to bait those models out or tempt your opponent in to spending focus or fury.
Charge/slam your own models. It's pretty basic positioning advice but if you don't know it you won't get assassinations to work.
Playing Against an Assassination List
Keep your caster back. Nothing protects quite as well as distance. Be aware of non-standard moves that extend threat range. I have seen a caster stay just outside of 19 inches away from siege only to have a heavy push him an inch and bring him into assassination range. Know the threat range of the assassination attempt and try to stop your opponent from being able to access key pieces. Be aware of how the assassination is going to come at you, there are tricks you can use to stop it. Caine2 will struggle to kill you if you are in cover and he can't use his rangers. If you opponent is relying on pushes to get the job done you can armlock with your beasts/jacks that have open fists (neither model in an armlock can be pushed). If you are playing Magnus2 you can have a Vanguard headbut him every turn (still love that idea). There are always ways to take the assassination out of the game, most assassination lists rely on you not seeing the assassination coming or underestimating the threat range of it. Camp focus or fury. As much as possible. Most importantly kill the threats to your caster as quickly as you can so you can get your caster back into the game. Most casters don't like being in your deployment zone for the entire battle.
I'll finish off with a list of assassination casters (almost certainly not exhaustive) and how they accomplish their assassination so you can be aware. I've included threat ranges, these are standard threat ranges (measured from the caster unless otherwise noted), you can increase them with things like push or slipstream. I've also listed the key pieces to make the assassination work so you know what you need to kill or otherwise take out of the picture to be safe. I'm also not counting casters who have a very good assassination run (Caine1, Lylyth2) but who's lists are generally attrition lists until the assassination opens up. These are casters you often see in builds that do nothing but gun for your caster right from the get go.
Kreoss1 - He'll knockdown your army and your caster and then shoot you to death (your army no longer blocks LOS).
Threat Range: 22"
Key pieces: Redeemers, choir, vassals,
Krueger2 - Telekinesis + stone double jump. In tier (as above) it will be a teleported WoldGuardian (but you gain Eiryss), out of tier it will be a primaled Ghetorix or Stalker (which is better but no eiryss).
Threat Range: 20" Teleport + Telekinesis.
Key pieces: 2xShifting Stones, Heavy beast with auto knockdown or reach (not colossal), Eiryss or Gorax.
Cassius - Combination of Hellmouth, often double hellmouth, curse of the shadows and the stone double teleport (Stalker with Primal). Don't feel safe just because the tree has no souls, he can kill his own guys to load up as he pulls off the souls during his activation not he control phase.
Threat Range: 19" (to hellmouth target within 3" of warcaster) 20" double teleport.
Key peices: 2xShifting Stones, Heavy beast with reach, Gorax, bone grinders.
Lylyth1 - Feat gives an additional dice on attack rolls, she can boost on top, shoot you, auto hit with parasite and then shoot again. Then Typhon can gunfighter charge and spray through to your caster from insane distance. She does need LOS to your caster but with SPD 7 and slipstream she can usually get it. Typhon does not require LOS.
Threat Range: 19" (21" Slipstream which she normally has).
Key pieces: Typhon, Seraph
Rhyas - Feat lets you place B2B with the first thing you hit, Rapport makes your scythean hit very accurately (MAT 8) and you can massacre it for another attack or wraithbane it to avoid defensive spells.
Threat Range: 4" from anything with 11" of a scythean (up to 20" with a colossal or battle engine).
Key pieces: Proteus or shepherds, Scythean, Nephlim Soldier or Naga Nightlurker
Seige - Rangers run into position to give a RAT boost, Siege drops foxhole to clear LOS, feats to increase damage output and shoots with a lot of boostable guns.
Threat Range: 19"
Key pieces: Rangers (only need 1), Defenders or Stormwall, Siege, Reinholdt, squire, Eiryss2
Caine2 - Rangers run into position to give a RAT boost and caine shoots your caster to death himself using his feat (he can kill a lot of casters without the feat too). In his assassination lists (he's also a great attrition caster) you'll also see gun mages and an Avenger to knock stuff down or push them out of the way.
Threat Range: 19"
Key peices: Squire, rangers, reinholdt, Avenger, gun mages, trencher master gunner, Eiryss2.
Makeda2 - Molik Khan. Leash, Road to War and Rush or Train wreck. he can move around stuff in the way and still make it to your caster with 5 fury (initials required for side step) and all boosted attack rolls from the feat as a POW 15 weaponmaster.
Threat Range: 22" (from MK)
Key Pieces: Molik Khan, Beast Handlers, Gladiator or Bronzeback.
Asphixious2 - It's the feat. You can still curse with your temporarily returned to life Tartarus. Again it needs LOS (but from 3" from asphixious after he has charged) and a charge lane but you are bringing 10 guys back to life, they can't all make it to the caster anyway so you can easily clear out a lane.
Threat Range: 24"
Key pieces: Tartarus, Bane Knights, Bane Thralls. Special note here, killing them does not prevent him from using them to assassinate you, you need to remove them from play. Especially Tartarus. Even this doesn't work as if you start doing this an assassination minded player will just kill his own stuff so he can feat it on you and that still works. Really you need something that prevents charges or takes you caster to very high DEF.
Vlad2 - Feats on Kayazy Eliminators and regular kayazy who killstroke. They charge straight through your army ignoring models and hit your caster at MAT 12 POW 14, 2 attacks, they can boost using your caster's focus or combo strike for POW 18 and two of them gain an additional dice on attack and damage discard lowest. The regular Kayazy have only 1 attack at MAT 12 but POW 15 and can all do the extra dice discard lowest spell. Oh and they are DEF 20 after they finish attacking.
Threat Range: 13.5" with 2 attacks on the caster or 15.5" with one attack, measured from the eliminators
Key pieces: Kayazy Eliminators
Magnus2 - It's a bad seeds list (though I believe this can be done outside of his theme forces and even with Magnus1). Renegades are the key. With rangers and the trencher master gunner you are RAT 10 and if the first shot hits the caster is knocked down (you may then be able to arc Calamity on to them as Renegades are also an arc node, if not Calamity then Obliteration). Renegades have arcing fire so ignore intervening models not within 1" and will happily shoot those models anyway to knockdown most casters. You then hit them with the other Renegade boosting damage (pow 16) and a boosted damage defender shot.
Threat Range: 19" (from renegades)
Key pieces: Rangers, Renegades, Defender, Trencher Master Gunner.
Ravyn - Known as "snipe feat go". Ravyn will feat to giver her mage hunter strike force boosted attacks and put snipe on them for extra range. They advance deploy, move 6" and shoot 16" meaning that if your caster moves 3" or more from the edge of your deployment they will pretty much auto die. They are only POW 10 attacks but they ignore anything that adds to defence or ARM and there are 11 of them boosted to hit. They also ignore LOS so you can't hide. You have to run.
Threat Range: 22" from MHSF
Key Pieces: Mage Hunter Strike Force
A lot of the best assassination casters also make very good (I would argue better) attrition casters but that's a topic for another post. Also there are finally pictures of Kreoss3 available, Alzer on the PP forums bought the model and assembled it.