I have a unit with shield wall. I'm running toward 3 units of archers. I know they are all going to fire on me.

Shield wall says while defending...

What is the exact timing on this? Can I wait for my opponent to roll his attack before I choose shield wall, or do I have to use it as soon as he declares me the target?

I'll I can find is that when he declares me the target, I'm the defender. At what point am I defending?

-Matt

While defending just means at any point during an attack in which you are the defender.

So, you could do it right after being declared defender, or you could do it after they roll their dice. I recommend the latter.

The first key step of the attack phases 10.2, the declare target phase. After this point, your unit is specifically the defender of the ongoing attack. The next important phase will be 10.5/10.6, after this point you know exactly how many hits the enemy has to work with (as well as potential MS and Accuracy). The next phase is 10.9, the Assign Hits phase. In this phase, the attacker will assign hits, which must be assigned to cause wounds based on the defense of the unit. Ideally, you want to wait until after 10.6 so you know exactly how many hits your basic defense will soak up, and wait to decide if you will need more defense to prevent most of the damage. If your opponent only rolls 3 wounds against armor 2, only one wound is getting through. If they roll 12, that might be more dangerous, but you might be able to reduce it by half if you add the Shield Wall effect.

Yeah I had a similar question. Usually, in other FFG games, they specify whether a card must be used before or can be used after dice are rolled (i.e., in Descent). The text on the Shield Wall is a bit ambiguous. Thanks for the explaination.