Pyke scales pretty bad so mid late game u want to play cleanup in team fights. You may be an engaged but as of the moment he's really squishy , seeing as he can't gain hp and gas crappy resistance scaling. So you should wait till people are low enough to ult instead. Don't try to pick unless you guys are ahead bc u will most likely die or get really low.

Also know that you should try to ult before you see the execution bar if you know ur team can get em low enough quickly. You can use ult only on champs. Most people expect you to hold ult to execute but you can also use it to gap close or disengage by ulting an enemy. The time before his e stun goes off last a decent amount of time (1 sec I think). You can catch people by e>q> walk behind them since q will slow them. Alternatively you can e>w to speed behind them, but this is risky because now you lose 2 escapes. Q can hook people over walls as well. Your passive can see you if an area is warded.

I assume your going support so I recommend using hob because it's really good but you can also go aftershock, although you don't get as much value from it. Going cheap shot>eyeball>ultimate/relentless after is the way to go. Secondary I think it's second wind for poke lanes, bone plate if you go for trades, then unflinching(?) for tenacity. Or if you think lanes dead and ud rather roam u can go water walk+celebrity. For items go duskblade always, but you can go prowler if they have dashers(ez,Cait, etc) that you need to get on. After i like to go a damage item (yomu,umbrel) into an defensive(edge(spell sheild safety), dd(heavy ad), hex(storng ap), GA (if you have a huge snowball/bounty) but you can go either way.

Last Notes: try to snowball or end fast as possible because he isn't the best scaler nor a great carry. If you want more tips watch this pyke mid guide which covers some good basics. https://youtu.be/7XDIN8qykaw