In the past, you seem to have had kings fightinging and dying in battle (like Harold in 1066). You also had them personally take command of armies.
But this doesn't happened these days.
When was the last time it happened? (And I suppose there are debates over where the line is drawn as to what exactly counts as commanding a battle etc. - obviously, heads of state/government still control warfare)
Do you mean whilst sitting?
If not President Eisenhower as Supreme Allied Commander, Europe during the 2nd World War is a likely candidate, though it was obviously before he was elected.
It also becomes confusing when you have a military junta acting as head of state, particularly if count fighting against insurgencies as "commanding a battle". I don't want to break the 20 year rule, but I can think of at least two relevant examples in the past five years.