Can you take more than one consequence to mitigate a single attack? If so, you're correct (assuming he chooses to take more than one - I tend to limit NPC consequences). But assassination is still possible...just takes more set up.
Yes, you can:
"Multiple consequences can be “stacked” at a time, combining their rating for the purposes of absorbing an attack. So instead of taking a severe consequence to cancel 6 stress, a player might take a mild (2 stress) and a moderate (4 stress) which would add up to cancel 6 stress." (YS204)
About ten maneuvers and declarations. 
Doable for an assassin planning a hit.
The major limitation to me is "must fit the situation". Time alone will limit declarations between the instant of injury and blacking out. I'd probably disallow any declarations based on preparation simply because of the short time casting is accomplished in.
Just one possible scenario (just counting up shifts, assuming the attack had no trouble hitting to begin with):
The attacker is a ghoul assassin sent after the victim by a warlock, and has Fists 4 (4 shifts), Supernatural Strength (+4 shifts), and uses his claws (+2 shifts). The Warlock provided him with a potion that grants him
Lightning-fast Reflexes (+2 shifts), and enspelled his claws to
Seek the heart of the foe (+2 shifts). The ghoul is also
Hopped up on steroids taken just moments ago (+2 shifts).
It's a ambush/surprise attack, so the defender gets no defense (-0 shifts), and the victim is unarmored (-0 shifts). The ghoul has placed
Got the hit all planned out on himself (+2 shifts) and
Caught flat-footed on the victim (+2 shifts). The ghoul invokes two aspects,
Mercenary Ghoul Assassin and
I kill for fun and profit! (for another +4 shifts).
That's a (net) 24 shift attack, which doesn't (in my opinion) rely on gimmicks like performing ten aiming maneuvers just before the attack; substituting and preparation in terms of a fairly small assortment of fairly accessible magical charms and a couple of non-magical ones (ie, steroids). Unless the wizard has an Endurance of 5, he's going to be taken out even with an average roll.
