In Korean media swear words are usually covered up by replacing one of the hangul with an "X" instead, which is also how it is done in Tower of God.
The sentence with the swear word is "이 미친 X이 진짜!!!", which is the censored sentence of "이 미친 년이 진짜!!!".
"이" means "this", which is used to direct the insult towards the other person.
"미친" means "crazy".
"년" is a word that degrades a woman, which in English would be "Bitch", with the suffix "-이" meaning "This", so it is "This bitch".
"진짜" means "really" or "genuinely", and in some circumstances "Authentic".
The full sentence, which is said by Androssi Zahard to Lo Po Bia Lilial Zahard, is "This bitch is really crazy!!!" or "You're a really crazy bitch!!!", which is a bit more spicy compared to the Line translation of "What the hell is wrong with you?!".