So whats the strategy for the incarnation of agony?

Do the debuff stages fall off faster if you just straight run away once you realize you have 5 stacks from nowhere?

Bring a life flask in case I randomly encounter it in 1/50 wildwoods?

Run away.

For the challenge? I haven't encountered her at the required level yet, but in lower levels she is beatable and will drop a nice pile of mostly useless unique items when you kill her.

Keep moving, run away and then return (using SRS helps there, she killed all of my other minions).

Depends on your build. You can just keep attacking and run away / heal up when you get a few debuff stacks, or simply equip warped timepiece.

if you are RF, just run away

my strategy is to immediately fall over in 0.1 sec before realizing incarnation of agony was there.