Mostly focus on the enemy tank unless an opportunity to grab a squishy kill pops up. By Round 3, with [Shared Siphon], you should aim to play almost like an off tank and hug walls/hold corners. Stay just out of reach of their tank's damage/speed boosts but close enough to hit most of your left click and just help win the tank trade. If tank is getting too much speed boost and smacking you all the time, swap in the [Booster Jets] item around Round 4/5.

If you have a D.Va/future mobile tank who is not really there for you to support and you can't frontline too much, change plans to start flanking a bit more and use the 2 optional perks for Rounds 3 and 7. Still try to melt the tank between having Wraith on cooldown if for no other reason than to just get more total damage, which gives more $ per round.

Don't forget to Wraith Form between fights so that your cooldown is back in time but you get a full mag of the 1st Power.

DO NOT use ult on the tank! Hold for squishies only (if at all) in the later rounds because it scales with Ability Power, not Weapon Power.