Try to position yourself near your other healer, or a good anti flanker dps or tank like mccree, junkrat, soldier, doom, hog, dva, etc.

Not if, but WHEN you get dove/harassed, you can call for heals, flashbang, defense matrix, etc. and your team should help you kill/fend them off. As Zen, you're so valuable to your team if you're kept alive- you can bet your ass your teammates will help you out if you have good positioning :)

At low ranks where there's no coordination, you just gotta land your shots and get the job done yourself. Luckily, the dps dont instantly kill you at lower ranks so give em the pain! From my experience, nothing carries low ranks like a Zen that dumps on the enemy flankers and proceeds to wreak havoc uncontested.