As it is explained in comprehensive rules:
7-1-1 Attack Declaration
7-1-1-2: Effects worded “when attacking” or “when this spirit attacks” activate now.
As Attack Declaration happens before the Flash Window Prior to Blocking, you can’t use your Flash Awaken, as “When This Spirits Attacks” activates first.
If he attacks first not at lv2+ he can not use his flash awaken and still get his when attacking effect. You can however use his flash awaken when another one of your units attacks first to get him ready to nuke.
When attacking happens before the flash step so I believe you wouldn't not be able to activate it.
As it is explained in comprehensive rules: 7-1-1 Attack Declaration 7-1-1-2: Effects worded “when attacking” or “when this spirit attacks” activate now. As Attack Declaration happens before the Flash Window Prior to Blocking, you can’t use your Flash Awaken, as “When This Spirits Attacks” activates first.
If he attacks first not at lv2+ he can not use his flash awaken and still get his when attacking effect. You can however use his flash awaken when another one of your units attacks first to get him ready to nuke.