Firstly, she is doubly enslaved. Once, she is enslaved by her king, Oroonoko's grandfather, who falls in love with her and for this reason, he wants to marry her. And in this situation, Imoinda shows her strength by keeping her virginity until Oroonoko arrives at the Otan, although she knows that if the king catches them, they will probably die.
Secondly, she is the person who tries to kill the governor by shooting a poisoned arrow when the slave revolt takes place. Although she hurts him in the shoulder, she does not kill him because an Indian woman sucks the poison. This in an heroic act because even not Oroonoko dares to do it.
And finally, she is a strong woman because she is the only one who remains with Oroonoko till the end of her life and the only one who never betrays him. She shows her strength again when she agrees to be killed by her husband for not being a slave anymore.
To conclude, I say that with these three reasons I think I have proved that Imoinda is not a weak woman but the opposite.