OK, armchair analysis of the club versus spear's special abilities, based on the initial values comment.
Att 6*6=36. Double 72. Versus spearman Def 5 (0 armor + 5 att); Max damage 12
Att 5*5=25. Ignores some defense. Clubber def 6 (0 armor =6 att). 66% of 6 is 4, so 25/4 is about 6 damage.
Tripling the above values means around 36 max damage for the clubber, versus about 18 max damage for the spearman.
If the Spearman strikes first, good chance he will reduce the clubber by 1 figure, which still counters with 24 damage, enough to degrade the spearman by 2. Spearman might kill the clubber again, but will die if the clubber hits again. Assuming hits, of course.
Sure, the Spearman can impale as well, which can mean an attack at range or hitting two targets, but no matter how you cut it, at 72 clubber attack is just massive.
Since players can usually outsmart the AI a bit in tactical, arranging for the clubber to strike first is possible, as long as units are separated by more than 2-3 squares. A first strike by a clubber has a good chance of instakilling the spearman.
So, the question is, if it is overpowering, how to reduce the club?
Reducing attack strength to 5 is possible, but after the first round I think spear has a significant advantage at that point. Sure a double strength attack 10 max damage is significant (possible instakill), but the followup rounds have both weapons about equal.
Reducing the double damage by some amount is easier to tweak. Don't like 80%? Make it 85%. You get my point. Dialing in a number that isn't double is certainly something that can be arranged.
My favorite idea is to give spears shields, but wooden shields aren't usually available at the game start (must research them). This increases Spearman defense from 0 to 2, for a total of Def 7. This reduces max damage from 12 to 10, which increases the probability that the spearman will survive the first crushing blow attack.
Giving wooden shields to clubbers increases their defense as well. From 6 to 8, Spearman will still be looking at a 5-6 defense, so a 15 (25/5, *3) attack should still be enough to reduce the clubber by 1.
The spear could be adjusted slightly, by increasing penetration from 1/3 to 1/2 or something. a 50% penetration would reduce the clubber's effective defense from 4 to 3, (or from 8 to 4 if shields were available from the start), hence increasing spear damage to 8 max damage. 24 damage could just possibly mean an instakill although more likely a 2 figure redection is likely... I'd recommend this 'bonus penetration' be a special ability, though, with a cooldown.
Lots of options here, should a change be needed!