I had a similar problem at first with the new army of monsters giving me grief. Now it is not much of a problem. Two heros with weapons can handle most early mobs, if you don't expand too early.
It is also important to fight the monsters in battle, outside of your city. Attack them, but once in tactical combat, stay back 2 squares and let them come to you. That way you get extra hits before they hit you. That is often the difference between winning and loosing. Also, be prepared in tactical combat to hit once or twice, then withdraw so the mobs waste the tactical points advancing. You get two or three hits, then withdraw a square when only 1 tactical movement point is left. Last, but not least, note where the escape squares are located. Retreat your heros off map if the battle is going bad. Better to retreat and fight again than to die. It also helps to put the sprinter shoes on all active heros.
I am playing an extreme game right now, large map, all races against me. I have killed off all but two races. I have 20 cities and am looking for a showdown with the Altar. I have 270 effective force versus 50 and 10 cities for the Altar, 7 for the Oredockers and 1 force and city for the other race that I am about to wipeout. That is in 276 turns. You need money and food to expand. So keep both in mind. Big armies are expensive to maintain. I currently rely on Heros for conquest. Garrisons to protect my territory, and have built up an $11,000 gold surplus. That is typical of my last 6-8 games at extreme levels too. Emphasize Technology over Magic. Specialize in either one or the other in each city. If you have two cities each 50/50 tech and magic, the level buffs are 50% wasted when both cities hit level 3, 4, and 5. But if One is Tech, the other is magic, then the 20%+25%+30% buffs are working on all of the magic in one town and all the tech in the other town.
One Hero with a good weapon, enough armor, and a horse can defeat armies much larger than he is in the game. I beat an army of 18 Guards defending the Capital of the Computer player with just 2 heros, leather armor, and mace and shields. One hero was healing and the other was using hit and run to wear out the computer army. Fun, but slow going at times.
EWOM is a fun game. But on my system it seems to crash to desktop (CTD) every 2.5 hours of play time. That will be cleared up as software development isolates the problems.
Sit back and enjoy.