If you target an opposing monster that is being affected by "Shrink", your monster’s ATK becomes equal to the opposing monster’s actual original ATK, not the halved ATK.[1]
If your monster’s ATK has been changed by "Rush Recklessly", etc. and you target it with "Nordic Relic Brisingamen", its ATK will be set by "Brisingamen" and "Rush Recklessly" will not reapply.[1]
Continuous Effects that change the ATK and DEF will reapply after "Brisingamen" resolves.[1]
If you target a monster whose ATK and DEF have been halved by "Blackwing - Gale the Whirlwind", the ATK will be the one set by "Brisingamen". After it wears off, the ATK will return to the original printed value.[1]
If you target your opponent’s "Tragoedia", your monster’s ATK will be 0.[1]