I've tried tanking EC on my shadow a couple of times now, and frankly I'm finding the mechanic on Zorn completely unreasonable.
What it looks like happens is that with no apparent warning (other than the rough health level of the mob I'm tanking), Toth does the jump, I get the debuff, and I die all pretty much simultaneously.
After about the 5th instadeath, I did manage to figure out that just spamming skills on Zorn is Bad. If I do that, I'll often see the switch happening in the middle of an attack, but be powerless to stop the next attack I already have queued up. Bam! ... instadeath.
OK. So no more spamming skills on Zorn. Slow and deliberate. But it doesn't really help much. You see, as a shadow here's my typical skill menu:
Double-Strike (2 hits over about a 1s animation)
Project (1 or 2 hits over about 0.5s)
Saber Strike (3 hits over 1.5s)
Force breach (1 hit at about 0.5s. 15s cooldown)
TK Throw (5 hits over 3 seconds)
Slow Time (2 hits over 1s. Why 2? beats me.)
Notice anything interesting? I only have 1 skill in there that does only a single hit. The instant I get debuffed, I take a bit under 5K damage for each hit, of course. I also only have one skill (fortunately the same one) that doesn't give Zorn an entire second to debuf me after I initiate it. That's also the only skill that doesn't inflict the big hurt on me multiple times if it happens. So I can stop attacking the instant the jump happens, and I'm still quite likely to die. I had at least one death where I was watching Toth closely, but he jumped while my finger was going down on an attack. Zap, dead.
So the only way I've found to deal with Zorn is, after the first few seconds, just stand there like a punching bag. If I'm feeling lucky I can put on a force breach every 15s.
This mechanic is so completely unreasonable that I'm thinking I must be doing something way wrong. How do other shadow/assassin tanks deal with this?