i was actually thinking the same thing
only person i know that has been boxing shamans is Valkry multiboxing in dungeons but only 4

he or she been posting a few videos here on the website aswell

https://www.youtube.com/watch?v=_f7SU_AQOXU
https://www.youtube.com/watch?v=zTy2EGnD12A
https://www.youtube.com/watch?v=Kc_ANa0FuiE

burst wise 5 ele shamans would just killed everything in a few seconds but i doubt they will be able to tank bosses,
if you have them ilvl 850-860+ you should be able to down bosses within 1 min on mythic +5 so only 1 earth elemental would work perfectly

going even higher in mythics might work with earth elemental having a 5 min CD and last 1 min before it disapears , so if can time it and cycle between 2-3 toons placing them it could be possible but then again it all depends on how much damage that boss does to your elemental