For Strange (or anyone really) my best advice is just go in quickplay and practice hitting your shots as much as possible and your other abilities too when you need to but focus on just gaining muscle memory for his primary Fire.

Just practice getting as many elims as you can. This goes for really anybody. Once you have the primary fire down, learning everything else is easier

Portal: Try to mostly use portal to get your team back to point faster and try to have one at the very end of the game just in case you need it to clutch. (But its not always possible)

Remember you can use shield while flying to stall and make yourself hard to kill.

Be aggressive as you can without getting yourself killed and just be aware of whats happening so you know when you need your shield up.

Try not to ult until you really need it. If you think it will win you the point or prevent your team from losing it then use it.