There's a simple strategy to follow after acquiring the giant's gloves. You will need a ninja, red mage or black mage (or any combination of those three) to make this easier for you.

When facing a particularly tough boss - namely Omega or Death's Head - stack the Temper, Haste and Sabre effects all onto a character with a strong weapon (Knight with Excalibur, Ninja with Sasuke, etc.) and make sure he stays alive.

Shinryu isn't so bad, seeing as how his defense is a joke. Put up protection against the elements and heal in anticipation of the flood attack, as the elemental protection does no good against it.

Omega is particularly difficult; even with an Ultima Weapon on a lvl 65 Knight with Temper, Haste and Sabre I can barely deal four-digit damage. Have the Rune Staff before you face Omega, and if you have a White Wizard, put Protera up and keep him alive to cast Full-Life. Omega's singular attack can take out a lvl 65 Ninja, Red or Black Wizard in one hit, and his beam attack will quash any less-than Full-Life remedies. In addition, put up NulDeath, so you can take a breather when Omega tries to quake you.