Second, click the control button at the top of the setting.
Third, look at the right side, you will see control butoons, then scroll down till you find a cotrol that says ''use weapon''
Fourth, at the right side, normally you will see ''L-mouse button'' but click on the ''L-mouse button'' than press the ''v'' button on your keyboard ( you can tries others key but I think the most suitable one is v).
Fifth, press the comfim button below. Than goto a melee match.
Sixth, when you are inside the melee match, u should see that u cannot use the left click to attack, instead u should press the V (or other u have chose) in order to attack.
Seventh, hold the button V(or other u have chose) on your keyboard and scroll ur mouse scroll up and down.
If u have done it correctly, u should be spamming!!!! (note: u must not have a grenade with u for tis glitch or it would not worked.)