How to get Decay?

I use team Enoch lead, Death Trooper, Night Trooper, Scout Trooper and TIE Fighter Pilot and sending them to Geonosian. Even if Enoch die I still can get even 27 Decay per battle. If you will lose a battle Decay you got still counts.

it's a pretty easy one. You just have to go against a team you'll lose to. If you kill them too fast, you're probably not going to trigger it.

I got 20 in one battle vs geos.  Kind of an annoying feat since you probably won't win the battle if you want to max out the decays.

I found a Carth Onasi lead team and full auto got this in 2 battles. I've heard it also works against NS teams. Best of luck.

The problem might be your 5th that you did not mention. However, if you are winning then you might not have any characters getting killed. You only get decay if a trooper other than Enoch is killed with a non-critical.

This feat is much easier if you have low gear on the team.

Geos is a great idea. 

I should add that remodding your Remnants with CritAvoid arrows is helpful. And, too, if you don't find Geos, Mon Mothma does swarming attacks which can rack up many stacks of Decay. (I think MM/Rebels has a slightly higher CC% though)