I'm going to talk about the caster I have most experience with, pKreoss. He is the caster people usually start with, as he comes in the battlebox. He is a good warcaster, especially for a beginner. That might be the reason he is in the battlebox. ;)
Lets get started with stats and equipment: His speed is little on the low side. It's usually enough, but not his strong point. Str is decent, this is complimented by a great weapon which I will get to later. Mat is great, he can hit stuff. Rat does not matter and def/arm is surprisingly low for a leader of knights. He has only one more arm and two boxes more than pSeverius, which is not much. Cmd is decent, but not great. He gets only five Warjack Points, which is one lower than average. At this point you might think: "what makes him great then?"
I will tell you.
His Focus is 7 which is a bit on the high side. His weapon is great, with reach, chain weapon and dispel. Reach is a great ability on a weapon, it extends his threat range and the area in which he can make free attacks. For a warcaster free attacks matter rarely, because models who get into combat with him rarely want to move out from his melee range. Chain Weapon is very situational, it's awesome against some models but most will not care. It's not something that changes the way you play him, but it's a nice bonus. Dispel is also situational. You can use it to hit your own models and get rid of those annoying debuffs. It would be great for a melee assassination caster but not that great for Kreoss, as he doesn't quite have the spd or movement shenanigans to reach other casters. P+S of this weapon is a good 14, enough to smash infantry but not quite enough to threaten heavy warjacks.Even though his model looks really 'knightly' and he has a huge flail, he is not a melee combat monster. He is not like Feora, Reznik or Butcher. He lacks the movement tricks and melee abilities to really kick ass. He is also quite frail, so you have to be really careful with charging. However, when really needed he can use his seven focus for some melee attacks and boosts, but it's rarely the most efficient way to use your focus. He is primarily a support/denial caster.
Spells: Cleansing fire is not used very often. It's quite pricy and with it's low range, you won't get that many places where it's a good buy. Immolation is similar, but it's not an AOE, slightly weaker but it's cheaper. You can use it with arc node to snipe those pesky solos and weakened warcasters. You can pull off a spell assassination with an arc node. If you pull out one focus from a wrack and get the arc node in range, you can put out four non-boosted immolations, three with some boosting or two fully boosted. High def/low arm warcasters have a good chance of dying from two fully boosted immolations, especially if you get the critical fire. Average boosted damage roll from immolation will deal 8.5 damage to a arm 14 warcaster. It's not something you can depend on to get a caster kill, but don't forget it. I would gladly sacrifice an arc node to get those immolations on his caster.
Defenders ward is just awesome. It makes units like Daughters of Flame or Errants really hard to get rid of. +2 does not seem that big, but jump from def 12 to 14 and arm 16 to 18 is huge. It makes errants shrug off most blast damage and even pow 12 (which is quite hefty for anti-infantry weapon) needs at least 7 with dice to kill them. And because you now don't care much about blast damage, your opponent also has to go through their buffed defense. 14 def is nothing remarkable, but it still jumps from easy to hit to "shit, I can actually miss these guys". Defenders Ward should be cast on start and upkeep from there on. Only drop it when that one focus is absolutely critical on somewhere else and recast it to another unit when the original target is down to a few models or destroyed.
Above I spoke mostly about units. That's because units are most of the time a better target for it. However, it does make 'jacks quite resilient. Ward on a Revenger makes one tough little arc node, heavy in cover with it can reach surprising def. Lot of people have commented on the forums that they use it on the Avatar. That's is definitely not a bad decision. Units just usually benefit more from both sides of the Ward, jump from 10 to 12 defense is not nearly as significant as jump from 13 to 15. It also protects from blast damage very well and blast damage is something units just hate.
Lamentation is great denial. This on Kreoss will create 14 inch zone where cost of spell casting is just crazy. It's also great against upkeep-heavy casters as it's also doubles the cost to upkeep spells. If you manage to get within 14 inches, they will have to make some tough decisions. Either pay two focus for each or drop them. They can still move out of your control range and cast spells at normal cost, but if they drop upkeeps, move out of the range and cast them again, they still have to pay the spells cost instead of just one focus to upkeep. Lamentation will mess a lot of plans, but you should not be overly aggressive with it. Your opponent can still just feed jacks with all of his focus and proceed to kick ass. Kreoss is also quite frail, so moving within 14 of enemy caster can be dangerous.
NOTE: Your opponent can use an arc node to avoid the double cost. The model actually casting the spell has to be within control range for this to work.
Purification: Highly situational. It will dispel your own upkeeps too, so don't upkeep them on a turn when you are planning to use this. It's great for getting rid of other players annoying buffs and debuffs, but the point cost is quite hefty. You have to balance this with how important you think your own upkeeps are and how much more effective your turn will be without his buffs. It's also great at dispelling those upkeeps that have a high cost to cast. Your opponent has to either stop casting it or waste a lot of focus every turn.
Last but definitely not least, the Feat. It sounds very simple. All enemy models in his control area are knocked down. You don't need LOS, it just knocks EVERY enemy model down in his control area. This is very powerful. It has both offensive and defensive side.
For the offensive part, it has two effects. Knocked down models never block LOS and are really easy to hit. For ranged attacks, their defense is reduced to 5 and you hit automatically in melee. The LOS part is huge. Your opponent can't hide his caster behind his models as pKreoss can just knock them down and shoot straight at the caster! Also, many casters depend on their def to keep them safe, this feat reduces it to a pathetic level. This feat makes your charging weapon masters really devastating as they will hit with every attack! You can also nominate your charges with more freedom as you don't have to worry about free strikes. One possibility is to run right past his troops to prepare for next turn. He can't counter charge them as he has to get up next turn.
There are many things that you can't affect with his feat and experienced players will use them to shield their caster. These are cloud effects, terrain, camping focus and simple distance. But this forces his caster to hide and you can still use the feat to whoop his other models asses!
So we get to the defensive side. This revolves heavily around the fact that when you are knocked down, you have to get up! Models who get up, have to forfeit either their action or movement. This means that they don't get to charge, aim or run. It also means, if they are not in melee range when they activate, they don't get to attack with melee attacks. This can help you to get in melee, you can knock his troops down and move close to them, without the fear of them attacking at you in his turn. However, if possible, you should aim to charge in the feat turn as it grants you automatic hits.
Note that his Warjacks and Warcasters can just spend a focus to get up. It's not as devastating to them, but it's still one precious focus for each model who gets up. That's 3-4 focus for a normal battlegroup just to operate normally! His ranged 'jacks don't care so much, if they are in range they can just forfeit movement and blast away. But they won't get the aim bonus.
With all that said, offensive part of his feat is much more important than defensive. The fact that he can't operate on full efficiency for one is more like a insult to injury. It's still great feat for scenarios and helps you get to the right position.
Models immune to knock down are obviously big pain in the ass for him. However, it's rare that his whole army is immune to it. If they are, don't get depressed. He has plenty of other tools. There are some effects that stand up the models for free and in those situations you have to capitalize on the offensive side of his feat.
Quick list of things to remember when playing him:
- Feat, feat, feat. You should start planning your feat turn from the beginning. With Kreoss, the feat turn will usually determine whether you rise to victory or fall to defeat. In most games you have to decide whether you save the feat for assassination or use it to crush his army.
- Deny. Lamentation is awesome for ruining his plans and you have two tools for dispelling upkeeps. Often you can ruin his whole turn just by upkeeping lamentation and being close! Don't forget that he can still just feed his focus to jacks!
- Cast and upkeep that Defenders Ward! It's almost always worth it!
- You can snipe too! With an arc node and seven focus, immolation can get rid of those annoying solos!
- He is not a melee monster. He can melee, but he is no Feora.
- You are really frail.
So what to get with him? One surprise for me was that he synergizes pretty well with ranged troops, even if he has nothing else for them than his feat. On his feat turn, they are just plain nasty. We have many powerful shooters who just have trouble hitting. Redeemers, deliverers and cleansers destroy pretty much anything on his feat turn. Fully fed Redeemer is quite capable of ranged assassination alone on his feat turn. He can either fire three rockets with one boost or two with damage boosts. It's great range compliments this well.
Don't still forget melee models. Autohits are great. It also provides your melee troops one turn where they can move freely with less fear of counter charge.
Should you get an arc node with him? That's a hard one. He has only two spells that need arc node. Immolation and cleansing fire. I think that arc node brings a welcome versatility to the list and I find myself sniping solos or damaged casters quite often. Revenger is more than a arc node though, it's in fact a great little jack. But at a 6 point price tag, he has some very hefty competition. You can manage quite well without him, as the feat of Kreoss makes killing solos quite a lot easier. For a tournament, I would probably use the points for something else than a Revenger.
Edit:\ TheEmu reminded me of the Guardian, who I totally forgot. It is true that when you take him, you get a good melee beatstick and somewhat needed arc node in a same package. But it's still a really expensive package... 9 points is HUGE and he doesn't quite seem that good to me. Flame pike has reach which is really nice, especially for a spd 4 warjack. Critical pitch is situational and you can't depend for it to even happen. He is also slightly tougher than other heavy warjacks, as the arc node grants two boxes. But like I said, he is pricy. But even though I'm somewhat doubtful about him, I think he can be worth it with casters who need arc node only rarely and can really use a heavy warjack. Reznik comes to my mind at first. But for pKreoss, I'm not sure. For 8 points I would take him, but for 9 points... maybe.
To summarize, pKreoss is a great caster. Amazing feat, nice focus and spell list makes him a good support/denial caster, but he has some melee potential as well. If you learn to use his denial spells and feat, you are well equipped to face most of the challenges thrown at you.