Moira is an absolute S tier pick in total mayhem. She can put out a lot of healing or damage, and can stall the objective for a long time if the player is good.
If there is a D.Va on the enemy team you will want to focus on coalescence more rather than spamming orbs. You should not use coalescence unless someone is far away and you can’t get to them on time, or there is an enemy ultimate being used which you can’t out heal. Tell your team to focus D.Va if they want to be healed more.
Junkrat will most likely save his rip tire for when you use coalescence. If there is a Junkrat, don’t use coalescence until his tire is used. It is also very easy to avoid rip tire on total mayhem since you can fade every 2 seconds.
You can 1v1 any enemy with Moira and win every time, except for Roadhog. Try to focus more on eliminating the enemy supports, especially Ana & Mercy since they are easy to target with Moira.
Make the most of your orbs when there is no D.Va, Moira is nearly unstoppable when her orbs are left untouched, especially if you have a small area to bounce your orbs around.
If the enemy team doesn’t have an Ana, Junkrat or D.Va then you can keep teammates alive very easily, especially if you have an Ana or Zarya with you.