@RZ - He only forgot up until the Index seal incident. He remembers the Alchemist event where he performed well by figuring out the true identity of the enemy's power. You can't just say that he is confused because the enemy is using "powers" or "magic".

Unlike the previous cases where he had to get beaten up first (and get information) before eventually defeating the enemy, full information regarding Accelerator's powers were given to him this time. This includes his own powers. Add to this a relatively long period of time between learning this information and the actual battle.

Then he forgot to calculate a plan..?

And you don't call that stupid?