The Eidolon Battle

"Odin... cut us a path!"

The battle with Odin in Chapter 4 (Vile Peaks) is arguably the most difficult one you've come upon in the game so far. Not only is it just hard staying alive, you have a timer, a weak partner with low defense (*cough* HOPE), and the task of filling up Odin's gestalt bar.

Suggested Paradigms (Light + Hope)

Preparations

Using Aegisol or Fortisol before this fight would be good. It would save you time (and you need to use every second) instead of having Hope buff your party at the beginning of battle.

The FFXIII Complete Official Guide recommends even changing the Battle Speed setting from Normal to Slow. I didn't feel the need to do this, but you may want to if you feel it will help you make better decisions.

Fight Strategies

Start the battle off with either a shroud or MED+SYN to buff the party. Then, use COM+MED most of the time to have Light inflict damage while Hope heals. When Odin uses his Ullr's Shield, switch to RAV+RAV for maximum damage. Once Odin comes out of shield mode, you need to switch back to COM+MED for the healing support. Use MED+MED if things get bad. Don't worry, you'll still be contributing to your goal even if everyone is healing. When you see the blue button flash onscreen, push it! It will be above your team's status on the right side.

Alternate Plans

My personal plan was quite weird and uncool, but hey I survived and bagged the eidolon anyway! First, I use Aegisol beforehand, to protect myself immediately once battle starts. Because I was dying easily, and had trouble filling Odin's gestalt bar before time ran out, I set Lightning as the permanant medic. Hope switched off between RAV and MED. This way, I could control where my heals went, how fast I healed, and fill up the Gestalt bar all at the same time. There were times when it was boring, such as me healing when there was nothing to heal, but it worked.

If niether the normal strategy above or my personal plan works for you, you need to backtrack and make your characters stronger. You can do this by fighting various enemy respawns and applying the CP to Light and Hope's Crystalarium. You can also equip them with Spark Rings (+lightning resist) and Silver/Tungsten Bangle (+HP).