I found a way how to shove my religion down Boudica's and Rastafari king's throat
their holy cities where forever converted and they couldnt do crap to keep their religion. No wars required.
Reformation belief: unity of prophets makes your religion incredibly resistant. Ive seen AI wasting their prophets repeatedly just to cut my religion in half. After that its only a matter of pressure to quicly bring your religion back in his holy city.
I solved this problem with Arabia, trade routes, religion pressure enhancer belief, and the grand temple (it applies to trade routes as well, so with with arabia it is very powerful). First i sent my GP to ethiopean capital. Boom converted his capital. Then send 2 trade routes to him for 145 pressure. His own religion had 30 pressure. He was never able to erase my religion, and even when he flipped his city back i still had almost half of his holy city population with my religion.
A few more turns and it was all back to my religion in his holy city. I never enjoyed myself so much in civ before
seeing hoe haile and boudicca holy cities were believing in my religion
This way i won the continent over two religionmongers with my super pressure + unity of prophets strategy.
Alternatively you can take the reformation that makes your missionary mini prophets, but it requires more proactive work. With unity of prophets you force him to waste valuable great prophets charges on erasing only half of your religion pop per use.