Aslamualeikum,
The word Amil comes into the urdu language from Arabic, and follows the weight of Fa'il from fa'ala. So Amil from a'mala. Amal means work or action, therefore in Arabic Amil (or whts transliterated by Arabs as 3amil with the 3 meaning ayn), means doer, executer etc.
However, when translating from urdu, historical and linguistic overtones need to be kept in mind as urdu hardly ever takes on arabic-words without adding its own subtle nuances in the meaning of the word. Since urdu and arabic are not even the same language family let alone the same language branch, arabic meanings cannot be taken as urdu meanings by default.
The word "aamil" could also therefore be the 'doer' version of the urdu verb "amal-karna" which means to cast a spell, or to infuse with some super-natural power etc. Therefore an Amil can mean, a spiritualist, or more specifically, someone who deals with the super-natural (the english equivalent would be witch-doctor). Certainly, in scholarly, literary and religious circles the word Aamil does not mean witch-doctor but rather simple someone who does-something, a worker, an agent etc -- however in colloquial-urdu and within certain circles of written urdu it also has the secondary meaning of someone who deals with amulets and charms etc.