I got up to about 1550 with 3x elemental shaman back when I was in deadly with titansteel guardians. I'm not sure how it would do now with full relentless - maybe 1800. It took a lot of games but was the easiest way to get the rating for all my relentless off-set pieces before I had a 5s healer. It's still possible to global people with only 3 shaman, especially since there are a lot less buffs and typically only one healer. RMP was the hardest to beat of course, due to all the CC, but some comps that are hard for real teams were easy for me (double healer + warrior).