the thing with monster is, if you ignore their lairs for too long, they eventually upgrade (even on easy settings) and start sending out larger and/or more powerful units. the undead lair probably would have been easy to destroy early, but once it upgrades to a cindercorpse lair and the single undead guys turn into 3 or 4 man units, they become dangerous.
also, did that undead army contain a dark wizard? those guys can kill off a whole army with their despair spell. bring some powerful ranged units and kill them before they finish their cast - or bring a mage with high spell mastery to counterspell it.
EDIT: or did the army contain a banshee? those things are immune to normal damage, so they can slaughter your whole army easily if you have no means of dealing enough magic/elemental damage to kill them
also, don't underestimate dragons and wildland bosses. in case you're not aware- you can inspect enemy unit before you attack them. look for abilities such as coal stones, physical immunity, overpower, fear and flame breath. these are real killers. doesn't matter what units you field, those powers can ruin your day if you don't know how to counter them properly.
