also, since they enjoy appearing together with those sirens that curse you with elemental weakness you may have caught that curse without noticing. triggering the poison cloud with attacks, summons or lolsummons works well, they seem to have a timer for those special attacks so it cannot be recast immediately
also, if theres a group of 5+ sons of lucian, chances are all of them cast the poison cloud when you get in range. invest in some +5 faster run orbs on your gear helps a lot with survivability, so the poison damage just ticks for 1 cycle which may mean that you survive. also, socketing some perfect turquoises until you are well above 81% resist may be helpful, theres a ton of poison damage dealt in worldstone keep and the mini bosses before baal deal a lot of poison damage too