It'll depend on the "frequency" of enchant compared to the rest of the spells.
See: https://forums.d2jsp.org/topic.php?t=38994239
Each attack has its own frequency. The chance that a merc will cast a certain spell is: attack_frequency / (sum of all attack frequencies)
I don't know the frequency of enchant, but the frequencies of inferno and fire ball are 60 and 30 respectively. The frequency of melee attack is 10, but only used in melee range.
So if we would assume the frequency of enchant to be 10 (and your A3 merc not being attacked in melee), then the chance it'll cast enchant is: 10 / (60 + 30) = 0.11 or 11%. If someone knows the frequency of enchant then we can calculate the chance your A3 merc will cast enchant with certainty.

Just tank while he engages, don't kill things yourself.
Demonlimb is also good enough for leveling people in norm

Slap his bitch ass and show him who's boss

dont hire the lightning guy