^^
[spoiler]
Actually, Negi already knows he can't block it, hes a dodger, and the girls are behind him, so evading the attack would get them hit and he has more hp. Instead he booted the staff off and thus was unable to dodge.
He simply can't outright parry an attack of this grade, especially if he didn't see it coming.
Part of the problem is that his opponents have passive defenses that are extremely hard to break through(the mandala shield, Rakan himself's ridiculous toughness), which requires a certain minimum of brute force to deal with. When his power is sufficient to actually hurt these guys, anything weaker is going to crumple like tissue. The other part is well...just look at all that iaiken going on, it comes with entering the major leagues.
[/spoiler]