This is part of an analysis of episode 6 of Death Note (from this page: http://samuraimoon.blog67.fc2.com/blog-entry-204.html#comment_list)
The author is criticizing Light for being a megalomaniac and making really stupid moves (「間抜けすぎるぜ」と間抜けすぎる行動を取り... (if it means that)).
In this paragraph he mentions Light's incredible ability of not reveling his faults/secrets (that he is Kira) even in situations like when he got surveillance cameras in his room as a reason for what comes next, but I can't make sense of that part.
At first I was trying to interpret that paragraph as:
Because Light has such great acting abilities if he wouldn't have taken any action (kill Raye and the FBI agents) even knowing that he was being followed by Raye, L should have no means to do anything against him (のに=showing regret) (implying that because of he's convincing acting Raye would conclude that he is not Kira (which ended up happening) and L wouldn't be able to further his investigations and end up suspect of Light (which also happend in the end))
But then I found this sentence:
見たこともない浮気相手に手の打ちようがない。 There's nothing to do against a cheater's lover that hasn't been seen
So now I think that Lにも手の打ちようはなかった instead of meaning that L should also be powerless (against Kira. L doing the action), means that Light should also be powerless against L (に indicating the target is L). But trying to make sense of that paragraph now is difficult. It would be something like:
Because Light has such great acting abilities if he wouldn't have taken any action (kill Raye and the FBI agents) even knowing that he was being followed by Raye, he(Light) should have no means to do anything against L (のに=showing regret)
This would make sense if the author of the blog is stating that Light just wants to confront L on whatever grounds as a way to show that he is superior. So he doesn't care about going under the radar but beating L. (Maybe that's what the author is refering to when he says 「間抜けすぎるぜ」と間抜けすぎる行動を取り). But mentioning that he's a great actor wouldn't be neccesary in this paragraph then.