In order to get a good combat agent, you need at least 3 command and a lot of martial. The agent contenders here are the baroness (free rats are the strongest agent killer tactic), the cursed (petrify is -2 atk and def and she gains +1 stats after killing her nemesis), and orc warlord in case somehow those are dead. Maybe some others could do it too if they gained stats but these seem like the best options to me.

The 3 command is for rats to fill out all of the slots. These will die in 1 hit against anything worthwhile, but that doesn't matter since they only need to take one hit anyway.

Combat works like this every round
Attacker - hits middle minion, or defender if empty
Defender - hits middle minion, or attacker if empty
Minions - hits the enemy minion opposite to them, or attacker/defender if opposite slot is empty
A chance for either side to retreat

All overkill is lost, so 15 attack hit will not do more than kill a single rat - this is their value. A goblin could fulfill the same role, maybe an orc can be useful if you already have the command for it but it probably is better to raise your martial instead of investing to get more than 3 command.

A special mention to corrupted heroes as they keep their minions even though don't have a way to restock special ones like knights and paladins. In this case, it may be worthwhile to be a little more careful to keep them alive and to rest sometimes to recover their health as they are powerful.

The 3 item slots to fill should be to gain as much martial as possible to raise the combat agent's attack. He only has one chance to hit the enemy if they have a middle minion before he takes damage, which means enough damage must be had to one shot the enemy when dealing with very powerful opponents who bring 2 knights and a paladin. The war axe grants +3 martial, but it often isn't easy to do this so the +1 martial axe will be useful too. Refreshing markets on a high prosperity place is a decent way to get war axes. Coastal markets free from unrest and etc are ideal.

If set up correctly, the agent who must be on the attack - this won't work so well when defending - will kill the enemy in the beginning of round 2.

At first you may be able to manage the menace and profile from killing heroes through laying low, but eventually an abyssal city from the deep ones will be needed to hide at since either too many heroes come at once or they will gain a cavalry escort which is too hard to handle and should be avoided. With time, almost all of the world's heroes can be slain and the new ones will be very low level and probably try to disrupt your combat agent when spawning - basically begging for you to kill them. The baroness with rat minions can kill far more rapidly due to not needing to rest or recruit new minions between battles.

One of the events for killing can cause 50% shadow in a relative, which is rather nice.