Finally take out the melee opponents and then last but not least the boss

Get to know your henchman, regarding henchmen temperament. You just have Airrivals Gold to get to know your henchmen. Take only a couple of them out at a time and see how they attack.

If possible, sit back, and let the henchmen do all of the killing (you will still need to choose the foes but don't kill them, let your henchmen do that).

Aside from skills, pick your targets appropriately. Your computer opponent wins, not by implementing brilliant tactics but instead by sheer brute force. It uses numbers to simply steam roll over you.

So take away its advantage. Once engaged in a battle, pick your targets wisely. Take out healers first. Next take out your soft targets who are usually, but not always the casters. They usually have less armor and die fast but also have the ability to do serious damage to your group.

Finally take out the melee opponents and then last but not least the boss. Again, you want to take away the computer's advantage, which is its large number of foes that it throws at you.

Eight level 22 summit heretics are much more worrisome to me then two level 28 bosses.