Im going to take a guess here and say grounding totems probably don't stack. I've never actually tried it myself but my guess would be..

The grounding totems only gives the party 1 effect once they are hit with a direct damage spell the totem giving the effect will go down and then youll have to wait for the next grounding totems tick to come in. Maybe lay down at different times? I dunno purely a guess as to how it would work.

I do 4 shadow priest myself. I havent tried 5v5 arena but I have done 3v3 arena. If the team hasnt just been formed and their rating is a good determination of their skill then I would say anything under 1700 I got a chance against. 1750-1850 I may kill 1 maybe 2 before I go down. 1900+ im so screwed its not even funny. I typically win about 80% of the games against teams under 1700.

I think Ellays 5v5 with him doing 3 mages and a warrior and paladin is probably a much safer route to take.