Taken from my guide here: http://www.xbox360achievements.org/forum/showthread.php?t=30...
I recommend having 100% battle trophies before this run, but it is by no means necessary.
Universe and Chaos are very easy, especially with the experience you have in the game now. I beat Universe in 9 hours and Chaos in 8 hours. I beat Satanail on Chaos at level 52, and you will too. I'm just writing a guide for Chaos because you can do the exact same thing on Universe and be fine.
-Before beginning be sure to have the Ashley jewel available by getting 100% monster data on Ashley. You will have to beat him 7 times, which should take all of 10 minutes at level 255.
-Skip cutscenes, always.
-Start off like usual. Open the occasional chest you come across. Use blindsides (the yellow circle does not appear on Chaos, you have to know when it would be around you) to beat the bugs easily and try to build a blue bonus board for more experience. Save before fighting Armaros of course.
BOSS: ARMAROS. A lot of people consider the beginning of the game to be really hard and Armaros to be a tough first boss. He's a joke. Play as Reimi and use the following method: shoot 3 shots, move a little farther away, rinse and repeat. Armaros will pay no attention to Edge or Faize, who will rack up damage over time. Eventually the armor will break and you'll be able to do some actual damage. Use Fresh Sages that you may have if anyone dies.
-Ignore PAs and head back to the ship and go to Lemuris. You can buy a better weapon before departing if you want.
-Sleep the whole way because PAs are pointless (this goes for the whole game).
-You received a Monster Jewel before you left. Transfer Ashley's monster data onto it and give it to Edge. Edge is now essentially invincible and will deal a lot more damage.
-Get Lymle and keep going. The second Monster Jewel is located in one of the northern chests on the ice lake I believe. Give that one to anyone. The DEF boost is invaluable.
-For every subsequent Monster Jewel (you only need 4) follow the treasure chest guide.
-Open any chests that aren't out of the way if only for the party SP. You'll need a lot.
-Blaze through Lemuris (literally) and move on.
-Zoom through the Cardianon Mothership and Alternate Earth, opening any chest not way off the beaten path. You should still be strong enough to decimate bosses with ease.
-Bacchus is worthless so don't use him.
-There is a chest with EM Bombs on the CM that you should definately get.
-Roak takes a while just because there's so much to do.
-Get better weapons if you haven't done so already.
-Blindsides will destroy Black Eagle if you're having trouble with him.
-Buy 4 HP Boosts in Tropp. Give one to Edge and Lymle, and hold off on the other 2. You need to get these pretty high up if not maxed out before Satanail, which is where the party SP comes in.
-Myuria is essentially worthless, but comes at a much higher level than the rest of your party, which has some merit I suppose.
-Make your way to the Purgatorium, the most annoying dungeon in my opinion.
-SAVE SAVE SAVE before Tamiel. I cannot emphasize this enough.
BOSS: TAMIEL. The hardest boss of the run. Your Ashley jewels have diminished in usefulness and Tamiel will not go down quickly. Kill 3 of his goons to leave 1 alive so that he doesn't respawn all 4. Then pound on him as much as you can. Once he starts using his laser beam attack, you're going to die pretty quickly. If you can whittle him down below 5-10k health, you've done good. You should be running low on party members, but it isn't over yet. Use your EM Bombs to deal the last of the damage. I literally was down to Myuria on both Universe and Chaos and used EM Bombs to get the kill both times.
-Onward and forward. Go to Aeos and give HP boost to Sarah when you get there and level it up a little, but save enough SP to max out Restoration.
-Your party at this point should be Edge, Lymle, Sarah, and Reimi.
-Before starting the Phantom Forces battle, level Sarah up one level to level 37. Your reward is Restoration. Max it out right away.
BOSS: PHANTOM FORCES. I would love to say there's a foolproof strategy to win this for you, but I honestly have no idea. Keep Sarah out of the way to heal and protect her. Use Fresh Sages if necessary. You will need them... a lot. Edge should be dealing the most damage so play as him. Odds are though it'll only be through critical hits. Be sure to have that skill set (maybe maxed). Good luck.
-Get rid of Faize and gain Arumat, who should be given HP Boost as well.
-Your final party is Edge, Arumat, Lymle, and Sarah. All should have Ashley jewels.
-Go through the Miga Insect Warren. Shed a tear as you pass the Dragon Roar chest.
-Armaros Manifest is annoying, but should be doable with blindsides.
-EN II is easy, just be sure to keep up on the treasure chests for the SP.
-Kokabiel goes down easily as well. Just go nuts when she regenerates or whatever. Kill the spawns right as they spawn simply by continuing your rage.
-After the fight, get the Laser Weapon by doing the white orb's quest. Give it to Arumat.
-Go to Nox, then go to synthesis for possibly the first time, and watch an unskippable cutscene.
-Synth a Light Gem (or something that adds the light attribute) to the Laser Weapon you just recieved. Also synth any spare Ashley jewels you may have.
-In the Palace of Creation, get the Holy Sword "Farewell" and ATK Boost for Edge. Max out ATK boost.
-When you get to the final save point, spend any remaining SP on HP Boost and attacks or healing spells.
-I was level 52 at this point, as mentioned earlier. You could be higher or lower, depending on random encounters and any choice to grind a little (which is not really necessary).
-Sarah and Lymle should have nothing on but healing spells. They should be set to "Stay Out of Trouble."
-Edge should have whatever skill you think does the most damage.
-Arumat should have Unholy Maelstrom set.
FINAL BOSS: SATANAIL. Easy peasy. Take Edge or Arumat and lure him to one side of the arena. Meanwhile, park Sarah and Lymle on the other side. Attack the orbs for a while. Edge and Arumat will die a ton, but Sarah will constantly revive them. When you finally break the orbs, it's showtime. Take either one and simply start initiating rush combos ad nauseam. The light synth on the Laser Weapon and the Holy Sword "Farewell" add to your damage output since Satanail is weak to light. Sarah will never die and you should take him down eventually.
-Plug in a wired controller for the ending and come back in 50 minutes. Interestingly, you should get Arumat's ending even though you did no PAs.
-Unlock the achievement!!! Congrats!!!