In the Season 6 finale of Game of Thrones, we saw just how far Cersei is willing to go to destroy her enemies and sit on the Iron Throne. Using the wildfire the Mad King had hidden under the Sept, Cersei took out the High Sparrow, the Faith Militant, the Tyrells, and members of the small council in one full swoop. She also had Grand Maester Pycelle killed by Qyburn's little birds, getting rid of all the members on the small council. Then, with Tommen's heartbreaking suicide, Cersei acquired the throne for herself though it's unlikely she'll be there for long. With Daenerys finally heading to Westeros, Daenerys will probably take the crown back from Cersei's cold, dead hands.
With a full fleet behind her that includes Yara, Theon, House Tyrell, and House Martell (technically the Sand Snakes, but they have the flags), and her dragons, Daenerys is ready to take back her throne. However, it seems unlikely that it will be Daenerys that actually kills Cersei. For one, there is Maggy the Frog's prophecy that states Cersei will be killed by her younger brother. So far Maggy's prophecy has been spot on so it seems unlikely that this part of her prophecy would be wrong.
But along with Daenerys comes her new hand of the Queen, Tyrion. It's much more likely that he will kill Cersei since he would fulfill the prophecy and he also just hates Cersei, as most people do these days. Honestly, with so many people looking to get revenge on Cersei, there may be a long line just to get to her but Tyrion will surely find a way. Or perhaps Jaime will be the "little brother" to do the deed? Either way, Daenerys doesn't need to kill Cersei because there are so many people that will happily kill Cersei for her.
And with Daenerys then taking the throne from Cersei, she then fulfills another piece of Maggy's prophecy that says Cersei will get the throne but a younger queen will then take it from her. At first it seemed Margaery (R.I.P.) would be that queen but now it seems pretty clear it could be Daenerys. Therefore Daenerys and Tyrion (or Jaime) would both be fulfilling the prophecy, bringing an end to Cersei's very short reign.