With fresh questing gear, 90s were taking out groups of 89s.
In some cases 10x 89, was not as strong as a fresh to 90 toon.

Not sure the issue is necessarily diminishing returns.



Your best bet for the Fear, is going to be a Round-Robin on the Tremor Totem.
Your guys won't be spread out that far, even if the fear is annoying.

You cannot check if something was successful, via a boxing software.
You can try to stack the odds in your favor.
For example put your Shaman in pairs, so even if one is resisted the other might land.
Then round-robin AB > CD > EF > GH > IJ.
Set it up as a ten step mapped key, not a round-robin within target group.
On step one, send your IWT to AB; on step three send it to CD; on step five send it to EF; etc.
On step 2, send your Frost Shock to AB; on step four, send it to CD; on step six send it to EF; etc.
Set it to execute on press or release.
On the even steps, set Do Not Advance for at least 0.2 or 0.3 seconds.

That means, you press the key and two of your guys will IWT to the target.
IWT is off of the GCD, so you'll be able to Frost Shock at any point.
Releasing the key will fire off the Frost Shock; hopefully one of the guys is close enough for it to be in range.
You can really spam the key, with the do not advance you shouldn't accidentally press it twice unless you were spamming multiple presses on purpose.

There's going to be diminishing returns.
Full duration > Half Duration > Quarter Duration...

Since the guy is a spell caster...
Round-Robin the Grounding Totem too.
Consider getting the glyph that reflects the spells back to the caster... with ten guys that's a totem every six seconds (if you were to spread them out, evenly).