I'm having trouble developing my legato technique, and I've tried all the lesson on UG about it. My fret hand usually runs out of stamina and I can't keep it clean and fast and end up bumping other strings. Also how hard should you pluck the string when doing pull-offs?
People are always trying to fascinate themselves with their own intellect.
your fingers should sort of catch the string as they pull of. this lets the next note ring out. and you should pluck fairly hard. as for the stamina, your hand will eventually get it.
^^ what he said. just keep at it, you will get it with time. you could try some exercises like for example: do 4 note per string chromatic scales but only pick the first note. try to make the others ring as loud as the picked note. and do it slow and controlled. do it starting from the index and also from the pinky. maybe even try doing it without picking the first note. but picking the first note lets you have a refrence point of how loud you want to be.

also, try to hit down forcefully, but with as little tension as possible. that may sound strange, but this will help with stamina. most people tense up when playing fast and that actually will slow you down and make you tired. so try to make it so that its pretty much the weight of your finger doing most of the power and then pull off to sound the note. most of the sound comes from the pulling off and note the bringing down of the finger. both will produce sound though.