Try holding the plectrum so that only about 2mm protrudes for picking strings with. This may seem strange at the beginning but you will learn to give the plec different length by rolling your finger and thumb after a bit of practise.
To get a pinch harmonic, you must play on the down pick. As the plec crosses the string, the side of your thumb brushes the string. Hard to explain without having a visual. Just keep tryng it on the same note for a while, pick/brush then vibrato. Do it a hundred times untill you get it then practise on every fret on all six strings. Oh yeah, make sure you have a good distorted sound.