does anyone know how I can get mythril crystal really fast besides buying them from moogle trade?

1. There are Grade missions that you can do that will give you mythril pieces. However, only two of them show up at a time, and are released by your level. IMO, this is the fastest way to get them, assuming you have the AP to defeat the heartless.

2. Subspecies Raid Bosses can drop any one of the four pieces, assuming your team is the last one who hits it.

3. Some of the story missions give them off as rewards

4. Leveling up when you're level 201+ give them off as rewards.

Well I'm a level 134 do you know how I can gain more exp fast?

Beat Fat Bandits in Agrabah. They give off the most EXP per AP.

 

But I seriously suggest you check to see if you have any mythril missions before you plunge into that. 

I did and thank you