it's difficult to solve this kind of problem as some of the words in Chinese have similiar pronunciation as well. The only thing that can be done is to memorize it. Phonics is good for unseen dictation as the child has more chance to dictate the words correctly instead of leaving spaces but if the child only depends on phonics instead of memorization, he will have a chance to write a wrong word with same pronunciation even in a seen dictation. Thus, beware of that.
For words with similar pronunciation, I think the key is we adult need to pronunce the words accurately, and then use phonics to explain to the kid why there is a difference in the pronunciation. Then let the kid read out the word when he/she write it during dictation practice at home.
If your child knows phonics, you can train her to look at the mouth shape of speaker so as to distinguish words of similar sounds. Pack and peck would be easy to distinguish by mouth shape. This is the trick for elementary phonics learners. Students having advanced phonics skill level can actually hear the difference, and consider the context the word is used (of course the speaker has to pronounce them right).