get lifesteal gear for kael and some masteries to boost him a bit (I went with offense + support trees: HP after first kill in wave, dmg output ...)
my lv50 kael was able to farm brutal 5.X or 6.X (currently lv 60, forgot what stage i was farming back then and if it was solo with 3 foods or 2x lv50 + 2 food)
2x offense + cruel sets, maxed skills (pulled 3x kael from ancient / mystery shards and topped off with tomes in about 6 weeks from starting)
right now my executioner lv50 with maxed skills (all skillups from farming duplicates, 3 in normal, few in hard, rest in brutal), tier4 masteries and lifesteal + immortal sets can solo brutal 12.6 (takes about 2m30s-3m and there is small potential of defeat on last wave, but I see no reason to get him to 60 right now)