There was some misuderstanding in forums about beheading men required by Executioner achievement.
Lets clear something out: Both Heavy knights are dealing blunt damage, despite of Agatha Heavy Knight's double axe, that is also does brain crushing, instead of beheading, so despite of the edge, its not able to cut through anything.
Heavy knights wields hand axe, which is able to decapitate foes, but the success of rate is extremely low, due to its damage %.

First find a flat surface to fight on, the combo best works that way, because your aim will be more steady.
CRUSADER
This is all luck-based but the best way to chop off more heads in my opinion is the Crusader's fine longsword, used as two handed weapon, without shield.
You can cut off the enemy's head once it's health low enough, so you need a combo.
-Pick crusader, Mason's sword worked best for me.
-Be sure you have plenty of energy or carry out a strong attack. Have at least 50% stamina.
-Aim for the head, and make a horizontal slash on it (Primary attack)
-Aim for the head again and deal a thrust attack (check your settings to find this kind of attack)

MEN-AT-ARMS
Executioner is also doable by Men-at-arms, pick sword with shield this time. Here, some combos:
Against Light Armor: Thrust on head/or chest, then horizontal slash on head.
Against Medium Armor: Thrust on head, then horizontal slash on head (x2)
Against Plate Armor: Thrust on head (x2) then horizontal slash on head (1 or 2)

For me about the rate of success was around 30% in both ways

Good luck