Damn you found out Adherbal's secret weapon;)
Seriously tho, I don't think a unit can fight 5 minutes;)
If the unit has high defense but very high attack , and the unit it faces is much worse, I wouldn't be surprised if it sustains low or even zero casualties (there is surely a greater randomness factor in the RTW engine). Adding to "slow deaths" is the very ineffective charging overall.
But strange things are a certainty sometimes...I find them often in my food as well:p