Opposer
OPPO'SER, noun
1. One that opposes; an opponent in party, in principle, in controversy or argument. We speak of the opposers of public measures; the opposers of ecclesiastical discipline; an opposer of christianity or of orthodoxy.
2. One who acts in opposition; one who resists; as an opposer of law or of the execution of law.
3. An antagonist; an adversary; an enemy; a rival.