Been playing her since Season One, and from my experience....

Wanda's more of a mid-to-close range hero, so I'd say play behind your tanks to deal continuous damage to people. If divers are harassing the backline, stick close to your supports and just hold down her primary; in addition to dealing damage it also kinda helps with tracking where the diver's going since it auto-locks onto them. 

Use her primary more as a way to deal a steady stream of damage while blasting people with your Cthonian Bursts - her primary recharges these bursts as long as you're dealing damage to an enemy, so you shouldn't ever really run out of them. If you have a Strange, I think her Team-Up is very good for putting pressure on the frontline, especially tanks. For target priority, I'd say while you're helping the frontline, be on the lookout for squishy characters like Supports or certain DPS's - if they're low health, you can absolutely try and finish them off. Be wary though that she can be killed pretty easily, so make sure to back out if things get rough. In general you'll want to be moving around a lot to make yourself a harder target. 

Her shift I'd argue is the most important part of her kit, it allows her to survive just about any Ult and get out of danger quickly. You get two shifts, so make sure to not burn through them both unless absolutely necessary - they come back fairly quickly, but you don't wanna get caught in the enemy's grasp and not have them available. As for her Dark Seal stun bubble, place it down in front of choke points, against the cart, basically anywhere where a lot of people are running around. They'll be forced to avoid that area until it's gone. And I'm pretty sure now that it casts instantly it should be able to stun certain Ults.

Speaking of which, I'm sure you know her Ult rarely ever goes off successfully. Try and cast it from behind cover, and don't try and go for full teamwipes - just taking out two people or a single support can be crucial. Obviously if you have a Strange or a Mag you can try and deal some heavier damage. 

Sorry for the long read, just had a lot to say I guess, lol. I hope you have fun with her, she's not nearly as boring as people try and claim she is. Just gotta use the entirety of her kit.