Use your ultimate to keep people away, the optimal time to use it is overtime on center.
in case of pushing the payload, you can put it in front of the cart.
If you are playing as a front liner and Seris casts her ultimate, you can put down your shield and prevent yourself and your teammates getting pulled in, and at the same time making her waste her ult.
When Makoa is stunned, aim a bit lower on him to hit his head hitbox. Once you learn the location of his stunned head hitbox you'll see it's very easy to output high damage on him during his being stunned.