All covenant features are disabled in MT so I don’t think the 10% stamina works in there as well.
Well I don’t think it does. I know it does. You can try it yourself. Swap to a covenant you have no renown with.
Just finished it yesterday on this blue geared paly with an heirloom weapon + post nerf crusader
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make sure youre using pots/food/rune/sharpening stones and wings + shield the opener to burst as hard as possible 
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get dbm its def 10000% harder without 
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make sure youre attacking either jarl or runeseeker at all times, with emphasis being jarl. I wasted the first 30 attempts hitting sygrin and not bursting the other two. Once jarl is down to execute range, its pretty much over 
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always save hoj for bladestorm. What made a HUGE difference for me was making target macros for all 3 so i dont waste time tab targetting 
Once i started hitting jarl/runeseeker instead of sigryn i got it in about 20 tries. Ive never even played ret till 2 weeks ago so good luck friend, if i can do it so can you 