"You must help me!"
要 is necessary, 一定 is optional. 你一定帮我 means "You certainly help me", which is something else.
So why is 你一定要帮我 not "you definitely want to help me"? It seems that 一定 is usually used for speculation ("That must be great") but not here.
I thought a cry for help, especially in emergencies is 救命 jiùmìng (救命我 jiùmìng wǒ - Help me!) They say it in all the Chinese Wuxia dramas!