>>531236553
I have a small crosshair for magneto, just a green cross.
The key is getting a good feel with your projectiles, at least enough to consistently chain 3-5 hits on a single target. Go in the range and set up moving bots, and then work on getting 4 hits in a row on each one from varying distances. Then do the same except you counter their movement (e.g. you go left when they go right)

Also understanding your Mag-Cannon's projectile speed is crucial. It has insane range, and I've sniped weak fliers out of the air with it.