My mate plays Orcs and as a Warriors of Chaos player (with a typically balanced list) I find the following difficult to handle:
Big blocks of orcs if they can get into combat. Even though knights and warriors are better, they will be outranked and ground down in a battle of attrition. Even if the Chaos player eventually wins the combat the unit will have fought for a number of turns due to steadfast and may have been severely depleted to the point of being combat ineffective. This is particularly true with knights. The orcs will be steadfast for at least 2-3 turns even if they get hammered and the knights will struggle to make their points back.
Black orcs. IMHO better than warriors, due to them being more expendable and with access to cheaper but still handy heroes. Also they have a -3 save modifier with great weapons on first turn of combat or 2 attacks with -2, either of which will cut through a fair number of warriors.
Doom divers are lethal against characters such as sorcerors who might want to wander off from units, or against units like knight and dragon ogres. Goblin bolt throwers can be a pain as well as they are very cheap and if they hit they can cause some pain to knight type units (any single knight or dragon ogre killed costs more than the bolt thrower)
I wouldn't bother too much with boar boyz though. They don't hit hard enough and won't have the ability to stick in a combat for more than 1-2 turns max before they are chewed up.
fanatics for obvious reasons, mangler squigs even more so. Similarly other cheap throwaway units like wolf chariots can be handy.
Potentially a unit like a giant as there is limited access to ranged attacks other than magic to cut them down before they get stuck in.
Wyvern mounted warbosses. With the talisman of protection so long as you avoid any chaos lords these are a real pain to bring down, except potentially with magic.
Points to remember are that the WoC units will cost a lot more than yours, particularly characters. If you play with any tactical finese, the WoC will really struggle to kill enough with any particular unit to make their points back.
Also remember the waaagh rule. You can use it to get in big charge moves and potentially to gang up on units. Unless they take a lord, chaos leadership is not great (although they will get panic re-rolls). If you can win a combat through ranks and flank/rear charges you have a fair chance of forcing them to flee. I woud be tempted to take some wolf riders for flank/rear charge purposes.
If your opponent takes sorcerors don't forget they have to challenge in combats unless there is another character with the eye of the gods rule in the unit. If not they should be fairly easy to take down with big bosses and ifthey do it is a seriously expensive unit ie. >600 points to be heavily targeted with doomdivers, magic, fanatics, rock lobbers etc.