Personally I don't get the hate this OVA gets. Imagine you have to make a stand alone anime with a 30ish min runtime and you DON'T have some super amazing story written by some genius. What do you do? You cannot have some drawn out character development or amazing world building. So why not just have some fun and go for the easy wins. Go cute girls with ecchi moments, guns, and simple humor. And tada, you have Katsugeki Shoujo Tanteidan. This is a mid show that is very light. Play it for some background noise while you do dishes or something. Good for a fewcheap laughs and ecchi moments.