The easiest way to get low level chaos shrine challenges is to use a low level party to beat them. The Chaos Shrine stuff is based on the average of your party level so try to use low level groups. The shards re given a bit random but be warned that certain shards do not appear unless the chaos shrine events are a certain level, so for instance getting Kain or Sephiroth is nearly impossible unless you challenging Lv. 60+ Chaos Songs.

You are using a pretty good group set-up just make sure Zidane or Vaan is leading the team since they have the highest speeds and most likely will get you to the tier 3 boss battle. Squall and Terra should be built for combat only. For the most part you need one character for the Map section (Bartz is my personal favorite) and then the rest of your party should be offense. You should stick to using Potions or Offensive items like Shurikens if you are having difficulty reaching the boss battles or surviving that section of the maps.