At the start of STL, there is a large group of low HP pell that need to be killed to start the dungeon.
These mobs are broken down into four groups. Each group has 3 pell.
The objective to begin the dungeon is to kill all 12. After doing so, you will get the challenge to kill 20 pell.
While I was soloing the normal version of STL for AMPs, I noticed that killing the 12 pell at the start gave 1/20 for the challenge.
Here's where the exploit comes into play. I pulled one of the four groups at the start. Killed two of them, then exited the instance. I went back inside, and they had respawned (since I didn't clear the whole group of three). I repeated this 6 times to complete the "kill 12 pell" objective.
Then, the dungeon and challenge started.
I continued killing and resetting to complete the challenge.
TL;DR:
Pull one group of 3 pell at the entrance. Kill 2 of them. Exit then reenter the instance, or use a combat-drop ability. Repeat until you get free challenge rewards.
Enjoy.