Yeap, my guild also runs with a shadow/assassin (me) and a vanguard/powertech and we always do it like this, when the two droids are dead (the ones with big purple cirle under them) one tank picks up the boss. Group kills bomber and takes down walker, same tank continue to taunt Kephess but after his jump he will put a dot on a player, just have the other tank taunt ASAP when the dot hits the first tank. When he jumps again you have the first tank again taunt the boss so he gets the boss. This way we always do it so the same tank always "tank" the boss and the other tank "takes" the dot, shadows/assassins are p bad against Kephess so we always have them (ie. me) take the dot and then the other tank tanking boss. If you dont have an shadow/assassin just have the best geared tank hold the boss and the other get the dot.
Actually, I don't have a problem (on my shadow tank) holding Kephess during this phase. He does hit very rapidly, which is usually a problem, but he actually doesn't hit fast enough to chew through Kinetic Ward. Since KW remains up, shadow survivability is just as good against Kephess as any other tank would be. With that said, it should *always* be a shadow taking the DoT if you have one. Not just for Resilience. Our internal/elemental resistance is substantially higher than the other two tanks, so the DoT hits for much less.
My group runs two shadow tanks, so we just trade off on the DoT and eat it with Resilience every time. Resilience is also super-nice for the leap, since you can just hang out right by the circle and pop it just as he's about to land (you can see his shadow right before he comes down). This also means that you will take zero damage from his melee ability which applies the DoT in the first place (otherwise a 15k hit) and you will be in ideal position to taunt quickly and prevent someone else from being pulled.
A voice over effect will play at some point during the Walker phase. This means the Trandoshans are going to spawn soon. We have our Scoundrel healer and our Vanguard tank (again, who is guarded by the Shadow, this is REALLY important) pull the first two warriors out.
We tried this in my group a number of times (even the guard swapping bit, which I think is pretty clever). Unfortunately, shadow tanks have healing agro that is more than sufficient to tip the threat scales in their favor. The Trenchcutters spawn with pre-set agro on the tanks (both of them), but the moment either tank generates even a smidgen of threat, everything will go for them. Since shadows generate AoE threat just by hitting things (when we self-heal), we always ended up in a situation where the "warrior kiter" had everything on him immediately.
Our strategy, and the one we found worked the most reliably, is to kite the entire second group. Tempest came up with this one, AFAIK. When the walker stands up, I pop combat stealth (to drop all trenchcutter agro) and move over to help kill the first two warriors. We have two melee DPS and a healer separate them so that our two ranged DPS can immediately AoE the cutters behind the walker. Once the second group spawns, I resilience + force wave to bump my threat just a bit and then start sprinting clockwise around the room. Transcendence helps a lot here. Warriors are pulled from my group as the DPS kiters have time, and then from the third group immediately on spawn. The third group of adds will spawn just as I run past them, and they usually die right around the time I make it around to the walker for the second time (we have our AoE DPS use their big hits on the first and last groups). When I get back to the walker for the second time, I pop Deflection and an AoE taunt. My co-tank pulls out one of the trenchcutters to delay the phase and allow us to center up.
We were able to use a few other strategies as well with reasonable success (e.g. focusing the warriors, then AoE; or kiting the warriors and tanking all three groups behind the walker), but they were all extremely tight in terms of execution and not very resilient to RNG. If the walker's single-target fire happened to hit the wrong person, it was generally a wipe. Kiting the second group just has the widest margins for error (at least for us), allowing recovery in case of mistaken execution or bad RNG.