I don’t really play El Dragon all that much, but I’ve played Phoebe successfully on nearly all the advanced missions. I find it’s very much about True Strike mobility, jab and move. That and, of course, landing criticals.
One of the things I find important in advanced story as a melee character is “turning” baddies, especially the big whoppers like MX Elites and Thrall Brutes. As you swing on them, pivot around so they’re forced to either show their back to you or your ranged friends. These guys have crit spots on the back, so with this kind of mobile flank you can ensure that at least one of you is working that spot. With her teleport and True Strike’s sidestep, Phoebe is quite good at this.
I still just prefer Reyna and Alani as healers on most advanced missions because of the increased offensive potential via scoring critical hits more easily and the fact their healing comes in bursts as opposed to ticks. Miko has very strong tick-healing, and its crit damage is very nice, so I feel it is a good choice too.