hey i’m not doing so great in further maths too considering my teacher is terrible at teaching… but i did go over complex numbers by watching bicen maths videos, doing the exercises and following with a bunch of exams qs from old spec and new spec and it has helped me so much. it instantly became my strongest topic. but if you need any help i can try my best to help you with questions ^__^
also, i have also done a similar technique with a few other topics such as linear transformations and it has been extremely effective.