Crawler has 16000HP and the Negator has 1000HP
Your first bet is to take out the Negator to make the battle much easier. Bring in Wakka since he's the only one who can attack Negator. The Gatling Gun attack is an attack to watch out for, it attacks multiple times and does about 160 HP of damage each time, so some of your less developed characters
can fall...especially if they are alone. Tidus' Cheer ability is incredibly useful in this battle. Have him cast it several times to reduce the damage of the Crawler. Summon Ixion for this battle since it's weak against lightning attacks. Use Thundara attacks constantly...but becareful as it will counter with Assault. Also watch for the Mana Beam, I doubt you'll deal enough damage before taking full impact, but as cruel as this may sound, bring in a summon you know you won't need much, like Ifrit and use Shield to take the damage for the party. HOWEVER, if the Negator comes back and Rikku steals, then the Mana Beam is interupted. Have Rikku use Lightning Marbles or any Lightning items if you have any.