The best option is what Telemain already posted.Originally Posted by duffybarracuda
The issue here is that Flameshock only does a little bit more damage than Earthshock does and only when allowed to run the full duration.
There is no macro that can be written or MOD create that can properly ensure that you will always use the proper one because it would have to check if Flameshock was on the correct target and only use it when it wasn't and Blizzard specifically disabled the ability to do something like that in a macro/MOD because they don't want the game making decisions for you.
Even then it wouldn't do the correct thing on the twins. Unless I'm mistaken the duration you have to burn down the shield on the twins is short enough that you would do more dmg while the shield was up using only earth shock when switching to the other twin rather than starting with a Flame Shock because the shield will be down or the heal will have gone off before enough of the dot ticks happen for Flameshocks total damage to pass Earth shock. Sure it will still do slightly more total damage but it comes after it really no longer matters.
As I said previously if you aren't willing to actively pick the correct shock to use then just use earth shock all the time. The DPS difference from mixing in flame shock isn't huge and doing it wrong will actually lower your dps. Personally I haven't added Flameshock to my priority list or action bar again and just stick to earth shock.