Tank should be picking up the bees and DPS killing them ASAP since their sting does a ton of damage and would likely kill most people. There's really not a whole lot a healer can do if people are dying to Doom or if no one is touching the bees.

As for Titan HM, his attacks follow a pattern so if you're slow at getting out of plumes consider starting to move before they appear. Your main priority should be yourself not dying or getting knocked off the edge, even if it means you need to stop casting a heal and move. After that you can start looking at why other people are dying. Unless people are dying during stomps or the tank is falling over from mountainbusters, then it's probably not a healing issue. Even without a cooldown up, a tank should never die to mountainbuster as long as healers are on the ball (although ideally they should be using one).