Since the late 2015 update, killing most cc troops is easy for TH8 and above. POISON. Two will usually do. Pull the cc troops out, drop a poison, then a second. Gone! This is the most efficient way as you lose the fewest number of troops possible, thus saving them for your attack. Otherwise, look at strategies below.
In clash of clans, the troops that come out of a clan castle to defend the village or war base are programmed to only attack one unit at a time. Well, that is if you gather them together. If you allow them to spread out, they will attack multiple targets. That is why it is critical, with some exceptions, to get them together and destroy them early. The same holds true for heroes, obviously.
So what is the best way to kill clan castle troops? Well, that depends upon your TH Level.
TH10 & TH 9: Two poison spells are the easiest way to kill CC troops. Witches are a great way to kill cc troops. Witches produce skeletons. ALL of the cc troops target ONE skeleton. What a waste, right? Most use witches in combination with barbarians, wizards, or archers. Minions work great too as they go right at the troops, keeping the witches back to let the skeletons do all of the work. The skeletons serve as a distraction. The CC troops attack the skeletons while your other troops can kill them.
TH8: Depending upon your troop availability, you can use the TH9-10 strategy or TH7.
TH7: Most attacks during war will be with dragons and I do not recommend even thinking about clan castle troops with an all dragon attack at this TH level. I’ll repeat that. If you are doing a dragon attack on a base, do not worry about the cc troops. Forget about it! Your dragons will take care of them. You can of course drop a poison on them, but you should not lure the cc troops out–just start your attack. For ground attacks, you can use barbarians or, better yet, a giant to soak up hits while wizards quickly wipe out the cc troops. Archers can be used or minions, which I don’t recommend if one suspects as air assault. To defend against air troops, such as dragons, minions, and balloons, a ring of archers works great, although wizards would work well too.
TH6: Many will lure out the cc troops and use to use a lightening spell against ground troops. Well, no more since the early 2015 update as lightening does not work on troops any longer. Usually barbs (cheapest and very durable) and archers is all that you will need, but one can use barbs and wizards as well. I never like to have wizards attacked as they have relatively low hitpoints. Archers do as well. I would rather a different troop soak up damage (i.e. let the cc troops target the stronger defense troop), while archers or wizards clean up. When the opponent has a clan castle that can hold 20 spaces, always have anti-air troops ready–i.e. archers and/or wizards. Have the dragons target tougher ground troops (barbs or even a giant), and use wizards or archers to take them out. In reality, nothing really hurts a dragon unless you have Level6 wizards.
TH5: Same as TH6, although you will rarely attack a base that has a clan castle that can hold a dragon, although minions are possible. Always have archers on hand for this reason, especially in war. Alternatively, wizards will do just fine.