Ser Robert Strong is an interesting character because he is effectively a robot operating with one rule: Protect Cersai

Typically one-dimensional monsters robots can not kill a high-level character like a King, Queen or Queen Mother and it takes a combination of characters scheming whose collective powers bring down someone like Cersai.

So there needs to be several characters/events taking place all at once for her to die. For Ser Robot to kill her I suggest:

1. Qyburn, Robot's controller, has to die
   
2. Ser Robot, as a monster robot weapon system must have physical proximity to Cersai- which he has usually all the time, and a chaos situation must develop
   
3. Ser Robot has to have a conflict in his understanding of his main directive (to protect Cersai), so as to not know how to act - so this could be "To protect Cersai I must kill Cersai"
   
4. Outside events, such as those generating magic need to occur, and those events need to involve someone who is aligned against Cersai, such as Tyrion. So if Tyrion does something magical, like ride a dragon, that might provide enough total points to align against her for her death
   

I don't see Ser Robot killing her but it is possible. I would suspect that the swords forged out of Ned Stark's ICE are used - Brienne has one sword "Oathkeeper" and I think Tommen has "Widow's Wail" so there could be something there. Also, the Valyrian steel dagger Littlefinger used to attempt Bran's murder is still out there somewhere. Cersai's death most likely will be at the hand of someone very close to her, like Jamie or Tommen. High level Queen Mother should be hard to kill.