The spire/yellow beam kiters relly do need to be healers. DPS give up way too much dmg doing it. Also .. they are 100 percent avoidable. The only other thing I can tell you as everyone does things a little different is that the majority of healing is on storm caller side. It takes 2.5 healers on storm and 1.5 on fire. This means that one of the fire healers has to be healing both sides. As far as the shields go if your people line up te shot properly they will take trivial dmg from the knock back. The instagibs you are experiencing is from not landing in the shield and getting hit with the first volley. All CC works on the shield phase I recommend stuns like the lightning sorc shield explode or flash bang. Also as a tip keep in mind storm side will always have a middle shield. This means you can send one group always to it and the other group can float between front and back.
We had a sage DPS as one of the runners and his DPS was fine (hovering around 1700).