General rules I live by for ULT and in general:
-Hunter/Ranged (personal preference)
ATA > ATP > 1600 HP
Hunter: Charge Mechs/Calibur/Diska/Raygun and you can solo clear episode 1 easy and comfortably. Most of these items are free or cost meseta. Your first actual purchase should be a Smartlink or a Heavenly Battle, then Arm Units.
Ranger: Charge Mechs/Raygun/Shot ASAP and you can clear most of the game honestly. First Purchase should be Heavenly Battle/Ranger Wall/Arm Units.
If you're a HUmar/RAmar and no FO is present, Shifta/Deband your allies, and Zalure/Jellen Bosses/Mini Bosses should be top prio, then Damage.
-Force (personal preference)
Party Play:
More Gear Oriented. You'll want to shoot for Glide Divine and then a Shifta/Resta Merge. Your job is buffing/debuffing and throwing out the occasional Rabarta/Gifoie to help manage enemies (not to deal damage)
For Stat Prio I generally stick to 1600~ HP and TP%Reduction, items like Mother Garb +, Psycho Wand, Dark Bridge, and Adept. You'll likely need a X/Technique Unit as even the best players don't have full sets of level 30 techs. Though Lvl 27+ techs are cheap and common, and a God Technique will easily boost it to 30.
Some players prefer not to be resta'd. If this is pre-agreed upon, try to oblige. In Pugs however, just keep everyone alive. You're not likely to run into this scenario outside of RES groups, and who wants to play with those people anyway

(I'm kidding I love those guys)
Solo Play:
Highest Techs you can get your paws on > MST > TP%Reduction Gear > HP > Tech Boost
https://www.pioneer2.net/community/threads/list-of-tech-boosts.965/
Standard Boot n'Nuke. Episode 4 is your friend, almost every mob is weak to fire.
-With >1600~ HP you can survive every attack in the game.
https://www.pioneer2.net/community/threads/hp-guide.8269/
-Plan your Material Usage before you use any Materials. Min/Max Builds are generally newb friendly, and can hide a lot of crimes when it comes to how your character was built late-game until you have the PD's to afford a proper build. But USE HP MATERIALS, they are independent from your material build and won't potentially negatively effect/botch a character.
https://www.pioneer2.net/community/threads/definitive-material-plan-guide.6706/
-Take full advantage of your action bar. This is hard for DC/GC players to get used to, but being able to S/D Z/J Anti/Resta/Mate/Sol without using shortcut menus saves time and lives. This will free up room for more important abilities on your action pallet.
-Pick up Everything. Sell it for Meseta. Sell the Meseta to players. I got my first Red Ring this way.
-Set personal goals and don't look at your EXP bar. With most Section Id's, there's something worth hunting on every Hard+ Difficulty. Whether it's Photon Crystals and Merges to sell, those nifty Elemental Wands, Enemy Parts, or a high Hit% Diska of Braveman to keep for yourself. Hunting Sub Ultimate Items is a great way to breeze through the lower difficulties, granted you aren't obsessing over your experience points which can make it seem like it's taking forever.
https://ephinea.pioneer2.net/drop-charts/hard/
-Up late? No one to play with? You can Solo level easily in Warrior's Pride at any difficulty. Including ULTIMATE! It's how I got from level 80-120 when seasons first started and there weren't many parties.
-Addons are a thing, and they're great.
https://www.pioneer2.net/community/threads/psobb-addon-plugin-lua-ui-addons.4543/
-Reshade and Echelon's custom skins and mods can make for beautiful and refreshing gameplay, I can't speak for everyone but it helps to keep me going.
https://www.pioneer2.net/community/threads/echelons-skins-modifications.4357/
https://www.pioneer2.net/community/threads/reshade-tutorial-thread-post-processing.14234/