Your best bet is to go for this when the enemy try to combo ults.
Save his Transendence for when the majority of your team is either
A. Frozen by Mei
B. Caught in Zarya's Ult
The enemy team will generally use this opportunity to fire off ANOTHER ult. This is your best bet to get the achievement.
I healed for 1650 damage by saving 3 people from a Mei/Hanzo combo.
You don't really NEED the combo, but it ensures that your teammates are going to easily get hurt, and it ensures they'll stay put while you're trying to heal them.
Zenyatta was recently buffed to heal 300 per second, and was made significantly faster while transcending, so this makes the achievement a bit easier. You could technically now get this by healing one person, although you're more likely to be doing some overhealing in there.
Note - Transcending can save you from being frozen, but not if you're ALREADY frozen.