Try do some Survival Mode with 5 others and go for the Wave 20 Achievement. The AI Raptors are far less dangerous compared to Player controlled, they are a VERY good way of learning how to properly aim a gun. It sure helped me a lot.
If you still can't hit for ♥♥♥♥, then you could get a faster mouse or increase sensitivity, so you can keep up with the fast turning dinosaurs.