There are 2-3 very significant coin flips that happen in a mirror match. Follow the strategy posted and that's your best chance for success. I took my BAM to R8 so unless I play the weak zombie effect and remove BAM's mods, it decreases the chances for a win on one of those coin flips.
There are times I lose 4-5 times against he same fleet. But then I win 4-5 in a row. It's a similar issue with GAC, if you lose a couple specific coin flips against the executor (whether it's a mirror or malevolence counter) you win or lose.
Executor is the main source of crystal income in the game for players now. I believe it determines the largest amount of wins of any character or ship in GAC (whoever clears it the most efficiently) and in fleet arena it's the meta.
I'm sure the TIE Interceptor will make empire fleets the meta though once we get it 7-star, just like how DB Fett is a game changer for TW since he's also a conquest character...