He wrote Monster Squad. 'Nough said. co-wrote. In fact he wrote the 3rd Act only, and you can tell because he has a distinct style, much like Tarantino with Crimson Tide. Your statement is as unfair as giving Tarantino sole credit Crimson Tide.
I saw Frank Dekkard and Shane Black do a live commentary for Monster Squad. They both acted like it was a combined effort, and Dekkard pointed out things that Shane wrote that were all over the film. Any time someone calls someone else "Ace" Shane wrote that scene cause, as you said, has a very distinct style and likes more hardboiled dialog. So, yes I was short changing Dekkard, but he wrote more than the 3rd act.