Hey Player you call alright from the way it sounds it appears you are facing Seymour Level 1... a rather easy battle if you know what to do...
First off your Team and their abilities
Tidus, Yuna, Rikku at First then switch in for Lulu
Tidus: Haste, Cheer,
Yuna: Nul-Flame, Nul-Frost, Nul-Tide, Nul-Shock, Esuna (maybe Shell)
Rikku: Steal
Lulu: Hopefully Level 2 elemental spells if you worked with her, Focus
Equipment
Rod of Wisdom you got in Besaid Temple (destruction sphere) (or at least customize a rod with Sensor)
First Strike weapon for Tidus you bought from O'aka (heavy price but a very nice ability for a speedster like Tidus)
Armor
The only thing that can really be dangerous is Confusion that those grunts may cast on you... so if you have any Lucid Armors (with Confuse Ward) from Drops availible equip them... they are life savers.
and now to the actual fight
Round One You VS. Seymour + 2 Guado Guardians
Alright for the first round The Two Guado Guardians cast Protect on themselves making them have double the defense... While Seymour casts Shell around himself giving him double the magic Defense... this is automatic and can't be stopped... Then Tidus should go first...
Haste Tidus, Yuna, then Rikku in that order then once Rikku stole from each of the enemies switch her out with Lulu
Seymour uses level 2 elemental spells in a pattern, Ice, Thunder, Water, Fire, then it repeats Yuna needs to cast the respective nul-elemental at least once before each of Seymour turns for the team to be protected. It is required you follow his pattern (Ice, Thunder, Water, Fire,) to remain protected
Meanwhile Rikku steals from the Guado Guardians this prevents them from using Auto-Potion to heal themselves.
Now once you have everyone hasted. Talk to Seymour, Tidus will gain a Strength Boost of 10 points, Yuna will gain a Magic Defense Boost of 10 (Wakka will also get a boost but I don't find him useful in this... but if you can use all the characters at least once and they will all receive the same amount of AP (2000 at least)
Now you may notice that if you go for Seymour directly his guardians guard him and take the hit and thanks to their protect status do less damage to them (and unless you stole from them they will use Auto Potion to heal themselves each time you attack)
This is where Lulu comes in switch her out haste her and use Focus for five times then blast each of the guardians with magic. They will soon fall. Once that happen hack away with Tidus making sure to have Yuna keep up her Nul-element spells each turn in the pattern (Ice, Thunder, Water, Fire) Once you get Seymour below 3000 Round two begins
Round Two
uh remember that big Aeon that Seymour summoned in Luca... well it's about to be summoned again... Anima will crush you with her Pain attack (as it causes instant death to one character per attack) unless you summon (preferably Grand Summon) your secret weapon a very magical defensive ice girl Aeon named Shiva. (Aeons are Deathproof)
Attack with Shiva normally until she in Yellow health then use Blizzara on herself to heal her... once you get her overdrive use it (and if you Grand Summoned her you get one for a first attack) but watch out if Anima gets her Overdrive shield to take less damage but once you use two overdrives Anima is frozen for good and KOed but now starts round three
Round 3
Seymour gets mad and increases his Magic stat and restores his HP... his shell is still in play so you got to use regular attacks or skills and he now doublecasts his elemental magic but still to the same pattern (Ice, Thunder, Water, Fire) starting with what ever comes next when you ended level one. Shiva will take the first blow and (unless it's Blizzara which would be intersting) collaspe sending you back into the fray...
As alway use Yuna Nul-elemental spells, Tidus hacks away and you may want to bring in Rikku for one last swipe (you can steal Turbo Ethers which is nice as they restore 500MP)
If you don't mess up on your patterns you should come out of this battle without a single death... (and if you deal at least 1400 on the last hit... Overkill)
If you have any questions I'll be right here...