This is relatively easy to get if you can 'lock down' a lobby in Quick Play.
Anyway, you want to have a bastion start to fire at you, and the moment you start taking damage from him, activate the Kinetic Grasp. This ability is the one where he holds a black hole in his left hand.
If the bastion keeps firing at you during this ability, without missing, he should get you between 350-390 shield damage. It's tight, but this is the easiest way to do this.
If you want to do this WITHOUT locking down a QP lobby, there's a few things you should keep in mind.
1) He can only absorb projectiles. He cannot absorb beams or actual melee attacks, so don't try getting it from Symmetra, Symmetra's turrets, Zarya, Winston, Reinhardt, Moira, Mei, or Brigitte.
2) The absorption doesn't last long, so the ONLY character who can get you enough by itself without ultimates is Bastion.
3) Ultimates you can get it from, if taking additional damage:
Bastion's Tank
Pharah's Barrage
Reaper's Death Blossom
Roadhog's Whole Hog
Soldier 76's Tactical Visor
Tracer's Pulse Bomb (You'll need to suck up the Pulse Bomb though, not get Area of Effect)
4) Ultimates you CAN'T get it from:
D.Va's Self Destruct
Doomfist's Meteor Strike
Genji's Dragonblade
Hanzo's Dragonstrike
Junkrat's Riptire
McCree's Deadeye
Mei's Blizzard
Moira's Coalescence
Reinhardt's Earthshatter
Torbjörn's Molten Core
Winston's Primal Rage
Wrecking Ball's Minefield
Thank you for your time.