Strategy working for me:
1. Send hero out to deal with little units.
2. Use phoenix strike whenever it's available - it will kill or wound the general's bodyguard, and piss the general off - send in your heavy infantry and cavalry to help your general kill theirs.
3. By the time their general is dead, you should have full units. Send everything up to their castle, and send your general back for a few seconds of resting if he's hurt badly.
4. When they spam units, either the castle will be dead too quickly or they will be easy prey for your next phoenix strike. Either way you win.