You need to assume magic items.
A party of 16th level characters without a sword of sharpness or something like it is a crippled party of 16th level characters-- at least the warriors are crippled without one. They can nick things to death, but as you say, the wizards could just use magic to kill it fast.
If nobody has a weapon with a chance of auto-killing, the next best thing is a weapon that adds to damage, like the flaming sword that does an extra 1d6 dmg. When I ran Palace of Shadows, almost every fighting pregen character had a flaming weapon.
However, what I really recomend is using Weapon Mastery rules from the D&D Rules Cyclopedia. It made it so that people could parry attacks, and that weapons did more damage when used by higher level characters.
I would modify it though, so that you only get a +1 to hit per mastery rank, isntead of +2 since the BtH chart moves faster in C&C than D&D (at least it does for warrior types).
Essentially, there are 6 levels of mastery: Unskilled, Basic, Skilled, Expert, Master, and Grand-Master. The chart below does not exactly correspond with the old D&D version, but is actually simpler, and makes more sense for C&C:
A weapon you are not proficient in counts as unskilled. You have -2 to hit and do half damage with an unskilled weapon (thus, a wizard could pick up a greatsword and swing it-- he just does half damage, and -2 to hit. Still better than punching someone!)
Basic skill is the "normal" to hit and damage.
Skilled: +1 to hit, and damage dice increases by 1 step (d2, d3, d4, d6, d8/2d4, d10, d12/2d6, 2d8, 3d6, 3d8, 3d10). So a person who is skilled in dagger would do 1d6 dmg, longsword 1d10 dmg, and skilled in greatsword would do 2d8 dmg.
Expert: +2 to hit, dmg dice increases by 1 step. So an expert in dagger does 1d8 dmg, longsword 1d12, and greatsword 3d6.
Master: +3 to hit, another dmg step increase. Master in dagger does d10, longsword 2d8, and greatsword 3d8.
Grand Master: +4 to hit, another dmg step increase. Grand Master of dagger does 2d6, longsword 3d6, and greatsword 3d10.
Training: in order to learn a weapon mastery, you must learn from somebody 1 rank higher than you. It takes a month of training. To become a Grandmaster, you must train with a Grandmaster, but it takes 3 months of training time.
When can you gain a mastery level? You can only become Basic Skilled in a new weapon (i.e. learn a weapon not on your class list) once you have a +1 BtH. You can only become skilled once you have a +3 BtH. You can only become Expert once you have a +6 BtH. You can only become Master when you have +9 BtH, you can only become Grand Master when you have +12 BtH.
I would do it the following way:
Fighters, Paladins, Knights, Rangers, and Barbarians gain 1 "slot" every level, but cannot begin using them until their BtH reaches +3 to become skilled. Example: At level 2 a fighter is basic skill in all weapons. At level 3 he is skilled in 1 weapon. At level 4 he can be skilled in 2 weapons, at level 5 he can be skilled in 3 weapons, at level 6, he becomes expert in one weapon, and remains skilled in the other 2. At 7, he can become expert in 2 weapons, etc. Note- fighters have an advantage here because their BtH is 1 higher than rangers, knights, etc.
Rogues, Assassins, Clerics, Druids, Monks, and Bards gain 1 slot every 2 levels. They can begin using them once their BtH reaches +1 in order to become basic skilled in a previosly unskilled weapon, and when it reaches +3 they can use them to become skilled. Note: Monk's unarmed damage cannot be improved with weapon mastery-- it is assumed that it is part of their monk training (which is why the dmg increases as it does already)
Wizards and Illusionists gain 1 slot every 3 levels. Note, that a wizard then must be 3rd level to become basic skilled in a longsword, and cannot become skilled in a weapon until 12th level!
Something else you might consider is allowing characters to use the "power attack" feat, even though it does not actually exist in C&C. That could increase damage dealt a lot as well.
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