ok then here is another extract from Gamefaqs, hope it helps.
18,000 HP is quite a lot. That's as much as both Sinspawn Gui bodies
combined. Needless to say, not only are you going to have to hurt him,
but you're also going to have to survive for quite a while. Against
what, you ask?

Actually, not very much has to be feared. He will use an attack called
Gatling Gun, which is about 10 random hits for 100 points apiece. This
attack is very similar to Blitzball Rush, in that it's annoying and
looks a lot worse than it is.

"But there must be a catch!" Well, there is and there isn't. Some
people might find it easiest to just attack him until he dies. My guys
did about 100 to maybe 200 points each turn. Sure, you CAN Attack him
150 times...

OR, more ideally, you can try to use Magic on him. A Thundara spell
does around 1,000 points to him, so you can just see how much more
quickly the battle would go. Problem solved...right?

Well, here is the official catch. Crawler has a little buddy. Negator,
who hovers high, prevents the usage of any Magic or Aeon. In the 50
turns you would be attacking him, you would then have to use Items to
restore HP - which would then later have to be purchased again. We
don't want that. So to use the magic, in turn speeding up the battle,
in turn saving precious Gil and time, the Negator needs to die.

Negator hovers far above everyone's head, however, and Lulu's Black
Magic is sealed, so she can't harm him. This leaves your options open
to Wakka's attack or Rikku's Use. Rikku can Use a Grenade to inflict
300 points to both Crawler AND Negator (yes, Use can hit this out of
range enemy), or Wakka could just attack for the same amount to
Negator. Either way, in three to four turns, he should die, and magic
should become legalized.

"So you just use Thundara and he just uses Gatling Gun and you win?"
Not quite. He now starts a three turn countdown to Mana Beam - a
magical, non-elemental attack that hits all characters for 1,500 to
1,700 points of damage! What can be done?

The answer lies in Scan. Check what it says above...or just listen up.
After magic is used four times on Crawler, he will bring Negator back
out. Since all Crawler does is countdown until Mana Beam, he'll do just
that. However, he won't be able to uses Mana Beam, since Negator is
out. So, if you use four Thundara spells before he can get his Mana
Beam off, it will be cancelled, and he'll have lost 4,000 HP. If you're
very fast, you could have Lulu use three, bring in stronger characters
to attack, and use the fourth right before he uses it. Take note that
after it says there is one turn left, he will immediately use the Mana
Beam.

"But I'm not that fast!" Then become that fast! Have Tidus use Slow on
Crawler, and chances are it will work. Have Tidus use Haste on Lulu if
you need to. If you need a little extra time, use Delay Attack, and
you'll slow Crawler down a couple turns. You SHOULD be able to get four
in before Mana Beam is used with a combination of these methods - you
are relatively four times faster with a Haste and Slow!

Be cautious, though! While Negator is gone, Crawler has a mean counter
called Assault. It will hit four 750 to 1,000 points, and is enough to
be dangerous. My Lulu was JUST able to survive. If necessary (most
likely), bring in Yuna. Just don't forget to use Haste on her to keep
up with Lulu if you used Haste on Lulu!

So there you have it. Kill Negator and use Thundara four times quickly.
Repeat this until it's been done a total of five times, and you win.

"But I just CAN'T spend that much time!" Well, it's not really that
much time, but you DO have another option. Once Negator is down, you
can also call your Aeons. If he's about to use Mana Beam, call in an
Aeon and let it use Shield. The Mana Beam will only do around 300
points! Better yet, Aeons stay in after Negator comes back - they just
can't come in while he's around, but they can stay! They hit for about
250 each turn. BEST yet, Ixion's Thor's Hammer Overdrive does around
5,000 points of damage to Crawler and 3,000 to Negator - a perfect way
to close the battle with a double Overkill.