I found the good rod and the super rod I also want to know want can I do..

You didn't get it, don't make me laugh. You get the Master Ball from Team Galactic's Leader.

Throw Quick Balls at Mespirit. Once the battle starts, it should be pretty simple, especially if you can put it to sleep.

Well, it took me a while to read it because it was confusing with the lack or commas/periods [*cough*cough*] Anyway, if I didn't answer your question correctly, don't blame me.

First of all, I'd like to mention that within the game, you are given the master ball without question. When you've beaten Cyrus in his HQ, he gives it to you automatically, and beating him is part of the storyline. So, maybe you've alreadying used the master ball?

Now, to answer what-I-think-are-your questions, you can do a lot more in the game rather than catch almost all the legendaries and obtaining the national dex. You've obviously beaten the E4, so you can also obtain the master ribbon in all 5 categories. You can also do many things undergound; play with friends, obtain secret base flags, etc. You can complete the national dex, minus event pokemon. You can also beat the Battle Tower 100 times in a row. Doing this 4 things will simply max out your ID. You can also do many more things in the game. Like, make a prized team, all level 100 with the best movesets they can have, beaten the E4 100 times... Don't just give up because the task is hard, keep trying.

Well, to catch Mesprit, try putting to sleep, and at night, encounter him and start using Dusk Balls. Using dusk balls at night have 4x the chance of catching a Pokemon, but Mesprit will still be a little hard... But not as hard as Cresselia, trust me.