Overwatch 2

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
or
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.

If you're having trouble timing the ult with other enemy ults, theres another method. If the enemy team is kind enough to use a Bastion, his turret can do well over 1500 damage during the duration of your Transcendence. I was able to get the achievement with one Bastion shooting my friend who was playing D.va, so good luck!

This was one of my final achievements needed until I picked it up without even expecting it by using it near a friendly BOB who was nearly killed as soon as he was thrown in. He stayed on the map for a long time as I brought him back from the brink and I think someone dumped an ultimate for chip damage against my other teammates. I still only had 1575, but this does make it more reasonable.