Hmm, I suppose they designed it as some sort of mobile guy, but frankly, I would just do the normal Magic Jar tank: Pick the largest/strongest monster you can find and posess it for awesome combat power. A build I designed used that on undead minions, instead of living monsters. He would put his body inside a portable hole sealed into the chest of his undead minion.
Use either the biggest monster you can rebuke (you should turn it into a Corpse or Bone creatures), or an awakened dragon corpse. Of course, since you can retry at will, if you play it safe you can try a couple of times to enter living monsters, too. Now this class is very anime. If you play it a certain way, you could never stay in one body long, and be virtually immortal. the Wildfire ability is obscene. OR you could create one body as a primary combat body, outfit it with magical items, and keep it somewhere safe, and only get it whenever you need it. The trouble is always taking your magic items from one body to the next. You might try to play with an evil version of Sacred Vow and take VoP, but it's doubtful that these effects would ever apply to your host bodies.
Now this is clearly a dread-necromancer class, even though it doesn't advance your rebuking level, unfortunately, so just use that. DN8/SSwtBE10/whatever. I would take a Martial class dip, too, since you should play this as a gish. I would definitely take the Stranger up until the return to Token ability, unless you always start from a rebuked, posessed monster. In that case you could maximise your rebuking by taking as many DN levels as possible, or other classes that advance your casting and rebuking.
so that would be:
DN8/Stranger1/Pally of Tyranny2/DN rest.
Race wise I would take a Bone creature Half-fey something (human if you can keep the bonus feat, and take Magic in the Blood, or something else with stat mods if you can't), or a Pixie, and drop the LA. You want your Cha to be obscene, and you need no physical stats whatsoever
. If you can only drop LA if you have RHD, then play a small Outsider and drop the LA.
However, to make a character who travels from body to body, just play an Imp Fiend of Posession. It's shorter, so you have more room for other stuff.