When he was first placed in Eden's body, Lukasz felt that his greatest weakness was his feminine body, as it lacked the physical strength and endurance he had become used to as a man. However, after becoming accustomed to Eden's body, Mantra has proven to be an able physical combatant, capable of wielding the Sword of Fangs with deadly effect. In addition to her 15 centuries of combat experience, she has also been trained by Aladdin in various martial arts techniques, making her an even more dangerous opponent.
As with other users of magic, Mantra has two primary weaknesses: a vulnerability to iron, and her true name. Her magic is seemingly powerless helpless against iron, as it can pass completely through her mystic defenses. She is similarly unable to affect someone wearing armor made of that substance, a fact which her adversary the Iron Knight has used to his advantage on more than one occasion.
Her second vulnerability, her true name, is less understood. When their true name is spoken aloud by a sorcerous adversary, a magic user apparently has their powers rendered helpless for a time, and are therefore left vulnerable to their opponent's magic. The amount of time this vulnerability lasts seems to vary, but can certainly last hours, days, or longer. Mantra's former mentor Archimage was captured when Boneyard used his true name against him, and was held captive for at least several months. When Boneyard used the same technique against Mantra, however, she was able to throw off the effect after several hours once Archimage tricked her into becoming angry to the point of blind fury, suggesting that strong emotion may play a factor. Presumably, her use of her mantra to focus her energies might have a similar effect.