iI will add some of my own insights on how the warriors' army works and how we can disrupt that.
As has been said above; chaos can outdo us in combat. not neccesarily in the "doing damage" part, but surely in the "soaking up damage" part.
because of this, it's very important to analyse how much pain each of your opponent's unit can take, and then make sure you have adequate tools to deal this amount of pain. just going one on one wont work, and many of their units hit back so hard that going 2 on 1 will still mean a loss of one of your own units. force overload is key when facing super elites (warriors/chosen/knights) and also when facing high strenght hordes (marauders)
to achieve this, the first thing we need is battlefield control: taking out his fast units/disrupters and ranged potential.
-taking out fast units: in my area most chaos players field at least 1-3 units of hounds, and maybe 1-2 units of horsemen. these units are purely meant to get their own units (alive) into favorable combats. it is our task to deny them those combats by taking out these support units. the standard core of most HE armies includeds 2 units of archers, or 1 unit of seaguard. These kind of units are exactly the reason why they are so important. dont waste you shots on trying to bring down 1 chaos warrior per turn, but let loose those arrows for the doom of his light units, and achieve the first step to victory: battlefield control.
-taking out his ranged units: while a chaos army is not reknown for its ranged potential, the units that are available to them are among the most dangerous in the game: hellcannons and (tzeentch) casters. the problem lies not only in their ability to do massive damage, but in their resistance to units designed to take out ranged threats. a hellcannon will laugh off an shadowwarrior/eagle/reaver charge, and most of their casters are too well-armoured/warded to be easy targets.
when facing hellcannons, i suggest sending in some fast and durable units to take them out, or at least keep them in combat to avoid shooting. this will usually mean small units of dragon princes/silverhelms or a character mounted on some kind of steed (eagle character can be very usefull here) if this is not possible. the other option(as always) is magic. pit of shades, purple sun, sig spells of both the death and metal lore all work great. even heavens has decent spells to take them out or avoid their damage. and if you're playing a defensive lore like light or life it will atleast minimise causalties taken, giving you more time to take it out.
on sorcerors: if they are in heavy units, just put those units 1 step higher on you priority list but other than that: make sure you have decent magic defence and weather the storm. if they are running around alone make sure to threaten them, but any player worth his salt will make sure their disc riders wont get charged. dont focus too much on them except if you have some spells to take them out easily (again death sig, metal sig are great).
about chaos knights:
while these can be considered a "fast unit" they are definitely not a light unit. most players play them in 5-8, and they can be considered equal to twice the number of warriors, but then faster. they are usually my first priority after taking out their chaff: when i take them out, my movement superiority will really shine, while as long as they are around my freedom of movement is restricted by their charge arc and range. they do have a 1+ save along with ws5 t5 and multiple s5 attack even when charged. this means they are no easy target for any of our units, but best to be taken out by a LOT of s5 attacks, or even better as many s6 attacks as you can get in there. its sometimes worth to offer up a small unit of lions to get them out of position and soften the knights up with their axes, but i usually try getting my helm bus + bsb (with gw) in there. and even they need support to remove them in one turn. A positive is their knights are usually on the flank and once you break them, you have all the freedom to encircle their lines.
about marauder hordes:
usually these units are 40-60 GW weilding loonies storming at you lines. if you dont deal with them, they will cause havoc. but on the positive side, they are t3, no save (maybe 6+ ward if tzeentch, but usually fielded as khorne). this combination of low T, no save and high numbers make them the ideal target for mass spells (My regular WoC opponent HATES flames of the phoenix
). and my only advice is "soften them up until you can handle them, then make sure you break them in one round" they are always the first (slow) unit to go down because the other units are so much harder to break.
my usual solution is: soften till there are less then 30, then charge in with a spear collomn (to break steadfast) and a combination of chariots/swordmasters/dragonprinces/anything with a high damage potential.
other support units:
chariots:
deal with them like any other chariot. either absorb the charge with a big unit, or charge in with heavy hitters. comparable dmage output to lion chariots but with higher T and save.
forsaken, monstrous infantry and monsters:
usually not taken by tournament players because of the following reason: while they can dish out damage, they cant take a lot. we as high elves have the luxury to field many high strength units which will cut through ogres/giants/shaggoths etc like a knife through butter. just make sure they dont hit you flank. if you field RBTs, these all make nice targets.
trolls:
we dont play with SC's around here but if throgg is allowed troll can become a real nuisance. they can take more pain than other infantry, and deal either multiple s5 attacks or anti-armour hits. try to stay away with your cav, unless you're sure you can kill enough before they strike. they dont like flames obviously, and while regen is good, its just as usefull vs s3 as vs s8, so multiple low S attacks can be very usefull to pick of some wounds.
units of chaos warriors:
there are 2 types: "blender warriors" and "tank warriors"
the first meaning multiple s5 or 6 attacks with "only" a 4+ save in combat (usually khorne warriors with halberds) and the second meaning 3+ save, 6/5+ parry save warriors but still with 2 s4 attacks. blenders are usually fielded in 18-21 and tanks in 15-25.
overall i think the blenders are more loved because they shred almost any unit in 1 or 2 rounds, and with their high initiative they can dish the damage before taking it.
luckily we have ASF. swordmasters and white lions, or a phoenix guard/silverhelm-block supported by heavy hitters on the flanks can take out these blender units pretty easily. just make sure you can deal so much damage that the blender unit starts losing damage potential before it hits back. try to get protective buffs on any unit facing them to the front, and getting units like swordmasters/chariots/dragon princes. engage them in overload or dont engage them at all. these units can be kept busy by fodder eagles/shadow warriors/archers untill they are ready to get slaughtered. so just keep them busy, try to get them out of position (they have frenzy most of the time) and engage them in turn 4/5 to remove them in 1 or 2 rounds.
tank warriors (this includes chosen)
these units can take so much pain that you should ask yourself the question "will i be able to break them at all?" if the answer is yes, take out the rest of the army then take them out with all you have left (+use magic to soften them up). of the answer is no, feed them fodder and never engage them. this is especially true for chosen deathstars which can be nigh impossible to break, expecially if they get 3+ ward and stubborn combined with bsb who alos has insane saves.
If you DO want to take them out both white lions and swordmasters do very well. tanks dont hit as hard as blender warriors (except chosen) so try to park high save units in the front (silverhelm-bus, dragon princes and phoenix guards, as well as well armoured characters are all options) and get as many lighter units (swordmasters/chariots/whitelions) in the flanks. mindrazored spearmen also work ofcourse
lastly:
flying combat characters:
chaos can field something very close to the (DE) unkillable fying lord. or else (in the case of heroes) they can make characters pretty resistant to shooting while still being able to dish out damage. dragon lords are rare these days (they cost nearly the full lord cap without gear, in anything below 2.5k, and sorcerer lords are very popular) but very difficult to take out.
magic is always an option to take out these nuisances but what other tools do we have? their speed makes them hard to catch (if possible at all) and they can quickly dispose of any of our support units (both combat and shooting units fear them) negating our most important tool; battlefield control. massive shooting can work but we also need that for other targets, which are easier to take out. they can charge small cavalry units, small elites and chariots on their own and win in one round. so how should we handle those?
well, hope i made some sense, and hope people are willing to add to this rambling piece
cheers,
J