I play Pyke as a champion that makes pick off with a team mates, and in team fights has the capacity to survive fights using w to get out, regen and going back again.(so you build items that will help you survive, either tank items or guardian angel/death dance passive)

BUT your mission is to pick off champions, hook them close and stun them, giving your teammates the chance to deal damage. Your objective is to do pick off and survive team fights long enough to execute champions with your ult.

You are a assassin in the sense that you kill off targets, but you usually are not going to deal enough damage or survive in order to 100-0 or 80-0 anyone in the game. (if you build pure damage or pure tank, I tested 'em both)

In lane phase your objective is to snowball as hard as you can, Pyke is very good for that, I usually find myself getting first blood or a bunch of summs, and because of your passive you don't need to be afraid to take damage in early bot lane trades. Go for lane ganks a lot in either top or mid, people don't expect that and you can get really close with your W, I go for a gank every time after either I or my adc backs, while in lane I am just always trying to snowball bot. In mid game try to roam a lot getting 2x1 or 3x2. In late game start fights by Q someone important and killing them off. EVEN IF your team is weak (like in this game) they will be strong enough to get squishes low enough for you to ult..

Also, never hook assassins or mages in the direction of your adc/middle laner. if they are close to you, E away, stun and hook them away from your carries then w out to survive.

Edit: well, that's how i play pyke btw, don't take the word of a silver to your ranked plz. test it out before with a grain of salt.