We have both healers stand in front of the walker. They both drop reviv behind the walker then just avoid saturation fire. This lets the healer assigned to healing the 2 tanks assist the healer on the dps doing warriors.
Here's a general diagram i made outlining how we handle that phase. The only real change is our healers stand more towards the walker's left foot as opposed to directly in front of him.
Kephess Trechcutter diagram
I bet commandos do help a lot there. We lose a mara every once in a while if he's getting smacked by a warrior (those dudes hurt) and then the walker shoots them. And if you notice the walker shots seem to have a 50% crit rate on them.
If you can consistently get through the trenchcutter phase though, all you guys really have left is making it to the 60% transition. Good luck.
Here's the unedited video
if interested. Not too much cut out, but could come in handy if you're looking for HP checks on kephess when he comes out. Mainly after the walker goes down, you really only want him to jump twice before gift of the masters. We had some issues on him if we let him get the 3rd jump off it since we'd enter the transition with a tank still bleeding from the debuff. It ended about as well as you'd think.