can someone please give me suggestions on when to / not to use his ult, who to use it on ext ty

Well someone's chasing you.

Silence them, put your dot on them, pop null zone and ult them. Ignite can also be safely cast while ulting without canceling it.

Team fight? Someone at low health? Ult them and get your team to focus them.

Buddy getting chased? Use it to suppress them. If your teammate is smart, they'll start whaling on them (if they can put down damage).

Use it whenever you can kill someone with it safely. In team-fights, try to use it on DPS, as they are hurt the most by the suppression.(you can usually keep nukers out of the fight long enough to swing it in your favor with a well placed/timed CotV.)

DO NOT use the ult on someone with a QSS off CD or if they have a hard CCer in range with the CC off CD.(you can use it on a QSS if you coordinate with other hard CCer's on your team to use their CC after your ult.)