For healers out there, who won't nessecarily be actively targetting the boss - set the boss as your focus target (shift + f I think by default), this'll place a mini version of the bosses bar on your screen, including his casting bar which will buy you precious additional time before the AoE bar is rendered, as well as alerting you to abilities with more subtle telegraphs (ie. AK's "impending catastrophe", Gogomera's "voice" abilities, the cyclop's "thousand onze swing" etc)