I'm not entirely sure it's even possible to do this as an entry level raid (talking 50 blues/tionese).
Kahesh does more damage than any of the other guys, and has 84k health. There's no way a tank can kill him in time (especially without support) before the reset-timer hits, and a healer can't do enough damage to kill him, and finally the dps can't survive long enough.
My guild and I oneshot this guy yesterday (hooray 1.1 resetting this boss fight for no reason) cuz we were able to help every 10s, but the new debuff is wrecking us.
We've tried many strategies. Highest DPS on him. Highest dps output healer. Best tank. Letting him kill you and then having the next guy he runs at pick him up (which randomly will and will not grant the debuff at times), have the highest DPS on him, make our dps kill 2 assassins at once and our healers all rotate healing on the highest dps every minute the debuff is off, etc.
Nothing bloody works on this guy. Did they even think about this when they increased the debuff? Every other version but 16m normal doesn't have him, and is easily doable w/o helping anyone else, but this... this is just insane.