The deal wirh Num Cavs is the combination of the free flanking 1 promo, base 20% withdrawal chance + the 50% bonus against melee units. That more than makes up for the 5 base strength against melee units (7.5 base) but does leave them a little weaker vs archers and horse units. You need to have a combination of C1/Flank2 and C1/Shock/Flank1 (9.25 vs spearmen does a good job) units. You need to whip/chop stables to give you 2 promotions to new units. F2 units weaken the archer defended cities and the Shock units serve as city-busters and melee stack fighters. Combined with the charismatic trait giving faster promos very soon you have C2F2 or C1/S/F2 or other specialized units that promote to knights, curas and cav. My current game I ran an HA blitz on Genghis, poetic justice there, and then promoted the leftovers to Cav giving me a huge force of 3 and 4 promo Cav that have run roughshod over Sitting Bull and Charlemagne. If I had had Hannibal and NCs it would have just been worse as I would have had more promos.