Omega and Shinryuu can easily be beaten at Level 53. Here are a few strategies:

Shinryuu: (SPOILER)The quickest way to beat Shinryuu, in my experience, involves four Coral Rings and eight Dragon Lances. The Coral Rings will eliminate Tidal Wave as a problem. Then have every character Jump with the doubled spears. Shinryuu should go down fairly quickly. Alternatively, if you have protection from water and ice, you can just hack away until it goes down. Dragon Lances, if you have any, are still useful.

Omega: (SPOILER)Omega is, in my opinion, much more interesting than Shinryuu. Omega has Auto-Reflect, is untouchable by normal physical attacks, and is immune, among other things, to Coin Toss and Meteo. My suggestion, unless you're looking for an additional challenge, is to make sure at least one character has learned !Sword up to at least level 5, and has learned SShot as well. Additionally, dual wielding from the Ninja is also advised. Change said character to Bare class, and use Sword and SShot as abilities. Equip the Ragnarok received from Shinryuu in the primary hand, and something like a Masamune or Strato in the other. Start off by casting Bolt3 magic on your sword, then follow with a few SShots (4x attacks). Omega should go down. I would keep the other three characters in a healing/support role; the Blue Magic Guardian is definitely advisable.

These strategies, of course, do need certain abilities. However, this is much quicker than adding 30 levels, and you may have learned some or all of these already.