Dragon Ball has been known to use transformations as a vehicle for strength and character growth, and one of those happens to be the Super Saiyan Grade 2 and 3 transformations. Future Trunks in particular learned the ability to use Super Saiyan Grade 3 during the Cell Saga while he trained with Vegeta, which led him to become strong and able to fight against Perfect Cell before he was defeated. That said, there are very specific details about Super Trunks as a transformation that must be understood. Part of that requires one to admit that the transformation isn't effective in the scope of Dragon Ball.