I'd consider the shammies going to a "click" macro.

The click will want to push three castsequences, and a normal cast.
The first sequence is Flame Shock, the second is Lava Burst, the third is Chain Lightning.
The normal cast (last) is Lightning Bolt.
It will ideally automatically cast Elemental Mastery and use your trinkets.

Flame Shock dot duration is 18 seconds.
Lava Burst is every 8 seconds.
Chain Lightning is every 3.5 seconds.
Lightning Bolt is every GCD.

Spam a key, repeatedly (ideally 2-10x per second) at a speed you can comfortably maintain for an entire raid.
Spam the key in word or notepad, for 10 seconds.
Then count the number of keystrokes and divide by 10.
This is your spam speed per second.

Now you will want a comma ( , ) per your spam speed, for every second of duration/recast delay.
So if you spam at 3 clicks per second, Flame Shock will have (18x3 = 54 commas).
If your spam speed is 2.4 clicks per second, Lava Burst will have (8x2.4 = 19.2 = 19 commas).
Etc... for Chain Lightning.

Valid "click" targets are....
/Click MultiBarBottomLeft........... The horizontal bottom left bar
/Click MultiBarBottomRight......... The horizontal bottom right bar
/Click MultiBarRight.................... The further right vertical bar
/Click MultiBarLeft...................... The further left vertical bar

And then Button1 through Button12

So create your Flame Shock macro.
I'll assume 2 clicks per second.
18 second duration.
(18 x 2 = 36 commas)
Macro 1 is:

#show Flame Shock
<Insert Assist Here>
/castsequence reset=combat Flame Shock,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

Make the Lava Burst macro the same, except for the correct number of commas for an 8 second recast delay.
Chain Lightning has commas for a 3.5 second delay.
Lightning Bolt can be the spell straight from the spell book, if you have an FTL system.
Otherwise it will need an assist option, depending on your set up.

Your "Click" spam button, will be on say "1" or "2".
You'll mash this button as your typical spam speed.
It will call your four other macros.

#show Lava Burst
/Click MultiBarRightButton1
/Click MultiBarRightButton2
/Click MultiBarRightButton3
/Click MultiBarRightButton4
/use 13
/use 14
/cast Elemental Mastery