I just beat the 3 table guys in Tartarus, but I tried to continue

Just clear until you hit the barrier before full moon. If you get rekt it means you need to farm some more. Should be doable in 1-3 nights

You should have personas closer to your level at this point. I don't really remember which ones are good so look up the persona 3 calculator and see what you can make. Make sure you use tactics and buff/debuff if you can

You really need to get fusing, the tables are level 13 and your highest level persona is level 10