To be more specific, the Demlich has immunities to all non-epic spells (cantrips through level 9), heavy physical damage reduction, heavy elemental damage reduction, and tremendously high AC. Plus 8 uses per day of a high-DC instant-kill SLA (soul gems), as well as the Lich's touch attack (which improves through Demilich levels). Sorry if I implied that it had immunities to everything...it's just...extremely durable. xD Not unkillable though.
Personal taste thing. Some folks like that.
Anyway, if I recall correctly, Lost Boy had pointed out in another thread that Ectoplasmic Shambler, when maximized+empowered+twinned, is pretty powerful since it doesn't allow a save, and it ignores Spell/Power Resistance. Though I'm not sure how easy it would be to fit all those feats plus the required Epic Psionic Focus (I and II) to accomplish that into your build. (I haven't used Ectoplasmic Shambler, personally. I tend to lean towards high-burst single target powers, or instant-kill powers, myself.)
Though, Tornado Blast -is- pretty great as well.
As for Shadow Body, yes, it does temporarily fade you back into the material plane after you do something to an enemy, however, it returns you to the whole "concealed+immunities+DR" state a round later, if I recall correctly...it's really not long at all. Sure, if you're constantly pew pewing, yeah, it won't be very solid protection, but if you stop, then it kicks back in...so if you time stuff right (assuming you're facing stock AI enemies) you should be able to still benefit mostly from the protection. Personally, I'm rather fond of Shadow Body for its immunities, mainly.
However, Iron Body also has some nice immunities. The movement speed reduction is an issue, but the other main penalty (arcane spell failure) isn't an issue for psionic characters (they are like divine casters, i.e. no spell failure in armor). Though not being able to use potions can sometimes create issues. But, there's not really a 'perfect' all-in-one defensive power, so you'll have to pick based on your preferances.
Actually, as a side note, it's not a bad idea to spend a few feats for armor proficiencies, or splash a level of fighter if the build has the room. Nuker in full plate and a tower shield is pretty nice.