Edrics Limited Response: After you win a power challenge as the attacker by 4 or more total STR, discard Edric Storm from play (cannot be saved) to initiate an additional power challenge this phase. (Limit 1 Limited Response per round.).

The thing that puzzles me is the part "this phase". Could that mean I can do it whenever I want in that phase as long as I am the active player?

If not, if I have to do the challenge at once there would be no action window before the challenge?

Dude, I asked the same question a few threads below. We should coordinate our board activities better. ;-) No answers yet, though. ~Is nobody reading my posts around here?

Nothing on Edric dictates any sort of timing restriction for the extra challenge, so you fall back on the normal timing of challenges. You can declare the extra challenge any time you want (if you want) that you would normally be allowed to declare a new challenge that phase.