What is a working strategy for defeating Yomi? Which paradigms should I be using, which monsters in my paradigm deck, and what is the overall flow of battle?
It's actually all a matter of surviving, you only need 3 paradigms really, and I only really used the 3rd one once. Have Noel and Serah both be Sabs and have a strong sentinel (Silver choc) for one paradigm and the other have Noel and Serah both be Syns and have a good medic (Green Choc or Flanitor) then when Yomi isn't in the aura stay with the Sab paradigm and keep casting everything you got on him, mainly poison (this is how you'll defeat him mainly) then when he retreats switch to the syn/med paradigm and cast buffs and heal. Just keep doing that til he succumbs to poison and wound. The third paradigm you may need is all sentinels and that's just for when he unleashes his special attack.
Chichu is ideal for this fight because he has the ability to take away Yomi's buffs with his physical attack. It even works while Yomi's barrier is up. I just switch to SYN (Noel) SYN (Purple Chocobo) MED as soon as he puts up his barrier and quickly buff, then switch to RAV RAV COM (Chichu) before the barrier falls to remove his pesky Bravery and Faith. With all Yomi's buffs gone and your party fully buffed, I hardly ever need a SEN. To finish him I just RAV RAV COM until he staggers then COM COM COM to own him. Keep MED SEN MED just in case you need the extra healing. I've been farming Yomi for Trapezohedrons for a day or so and I can take him down in under 5 minutes with this strategy no problem. Still only 1 Trapezohedron after 50+ battles though :(
If your low in level's with around 4-5k HP i would stick with a good first fight pack, serah-SAB noel-SEN "monster"-MED. take serah as a leader for a while till you get poison on him "spam it like crazy", then after you have it on him go with serah-RAV noel-COM "monster"-SEN. after he puts up his shield stick with serah-MED noel-SYN "monster"-SYN. going with this you should have him down to half HP if not more, but the fights not over yet "i find that he uses his one move after he hits half HP" the move can kill your entire party with one hit later in the fight so to prevent this use serah-SEN noel-SEN "monster"-SEN so as soon as you see it pop up switch it as fast as you can. with this you should be able to kill him with in the time frame.
later in the game after you finish the main storyline i wouldn't even try to farm him till your players are maxed out "easy to do if you max out a green chocobo ASAP for the bonus cp ability" and if you have the good weapons for serah and noel "again easy to do with a maxed out chocobo in the races" after you met these you can fight him with semi-ease, ok for the start of the fight start in serah- SAB noel-SYN "monster"-MED or serah-SAB noel-SYN "monster"-SYN, then when you hit him with poison switch to serah-RAV noel-COM "monster"-SYN with this pack you can keep attacking him and keep up with the buffs as he takes them off you. then after he brings out his shield go to serah-MED noel-SYN "monster"-MED/SYN "ether works. he will use his special move later in the fight so to avoid having your whole party killed switch to serah-SEN noel-SEN "monster"-SEN. after a few mins you should ether kill him or poison should kill him.
important items to have. "FIRST FIGHT" serah- 4 Delicate Magician's Mark noel- 4 Delicate Power Wristband
"POST-STORY" serah- Durable Collector Catalog noel- Ribbon
for each fight "first/post-story" i used all chocobos SEN-silver chocobo SYN-purple chocobo MED-green chocobo max out each and they will make a devastating party.