The truth, as I learned from tear and blood, all RDPS run from sniper. I vividly remember a solo ranked I had on Bastion in season 4 when ling sorc was way more powerful than it is now: it was a tank healer double dps match, with everything being the mirror class (PT tank vs PT tank, AP PT vs AP PT), except it was sorc dps (me) vs sniper. Eventually we won after 3 rounds of super intense fight What happened was our entire group decide to group kite that MM sniper, because nobody wants to touch it (everybody was, miraculously, 1800+).
Point of the story: MM sniper has unrivaled burst, with 2 autocrit ambush under entrench every 45 sec piled upon a 40k-ish single target burst that cannot be stopped once every 2 minutes. What all RDPS should do, which is what I always do, is to run... With all its power, MM sniper, unlike other RDPS, utilizes mostly linear kiting and positioning (can't turn around while rolling). Abuse that, as a merc or a sorc. If you are not a sniper, you should win by manipulation not fighting.