Friends wearing Nemesis greatly helped me get to level 28 on the previous boss.
My guess is "Nemesis dying way too fast" is only releavant for the higher levels, since IIRC on level 28 they were still taking/dishing out about 5 hits before dying. (which was much more than could be said from my Crius+ knight and only slightly more squishy than my Crius+ champion) .