If you want to do this quick and easy, create a new unit with average defence stats using your chosen model.
Build this unit in your campaign, or put it there from the start. Lay siege to the closest rebel/enemy settlement with an army of 2 units and make the hero unit the second one in that army (your don't really want a general in this unit).
When you attack, move the unit up to just within range of the defence towers, kill the unit down to 2 or 3men and RUN!, the other 1 or 2 men should be killed while running. Retreat from the battle and save your game. Exit and alter the stats to some thing like=

type greek immortals
dictionary greek_immortals ; Immortals
category infantry
class heavy
voice_type Heavy_1
soldier greek_myrmidons_elite, 10, 0, 1.3
attributes sea_faring, hide_anywhere, frighten_foot, frighten_mounted, command, very_hardy, no_custom
formation 1.2, 1.2, 2.4, 2.4, 4, horde
stat_health 15, 0
stat_pri 63, 63, no, 0, 0, melee, simple, piercing, sword, 25 ,0.1
stat_pri_attr area, launching, ap
stat_sec 0, 0, no, 0, 0, no, no, no, none, 25 ,1
stat_sec_attr no
stat_pri_armour 63, 63, 31, metal
stat_sec_armour 0, 1, flesh
stat_heat 0
stat_ground 0, 0, 0, 0
stat_mental 64, berserker, highly_trained
stat_charge_dist 30
stat_fire_delay 0
stat_food 60, 300
stat_cost 0, 10, 100, 100, 100, 2000
ownership greek_cities

(The Barbarian Invasion berserker skeleton really helps with kills)

Your 1 man unit is now a mean guy, and will only die if =

1. An elephant dies and falls on him.
2. He gets hit by a direct artillery shot.
3. You auto fight a battle with him in.

This technique will obviously not appeal to everyone, but is quite easy and has added a new dimention to my ,oh so unrealistic, personal mod.

I of course apologise if this is common knowledge (or just common sense). If so forget these words and we shall never speak of this again