THESAURUS
adversative, adverse, alien, antagonist, antagonistic, anti, antipathetic, antithetic, archenemy, assailant, bitter enemy, clashing, combatant, competitive, competitor, con, conflicting, contradictory, contrary, counter, cross, devil, disaccordant, dissentient, enemy, foe, foeman, fractious, hostile, inimical, negative, noncooperative, obstinate, open enemy, opponent, opposed, opposing, opposing party, opposite, opposite camp, oppositional, oppositive, oppugnant, overthwart, perverse, public enemy, recalcitrant, refractory, repugnant, rival, sworn enemy, the loyal opposition, the opposition, uncooperative, unfavorable, unfriendly, unpropitious
ROGET THESAURUS
Opponent
N
opponent, antagonist,
adversary, adverse party, opposition, enemy, the other side, assailant, oppositionist, obstructive, brawler, wrangler, brangler, disputant, filibuster, obstructionist, malcontent, Jacobin, Fenian, demagogue, reactionist, rival, competitor, bete noir.