The main issue was that they had void bone and ninja, all of which drain energy in some way. You had no overcharged heroes. Couple that with thier horses massive shield and archers damage and you're on the road to disaster.
Watching the energy bars, as well as looking for screen freeze effects, no whites were used for both fights, and although it probably was the drain of void in the first one, the drain of bone did nothing, as horse shield negated it and the initial bit was useless, as the enemies would not have lost that much though without seeing runes, i cant say for certain.
You on the other hand, lost tremendous amounts of energy at the start, putting you at an instant disadvantage, as not all your heroes were full hp, whereas theirs were