Grand Jewel is the guardian of the Dragon Block Staff. The party encounters it when Meru tries to pull the staff out of its head in Kadessa. Grand Jewel is a difficult boss, but it is not impossible to defeat it.
Lavitz/Albert: Just use additions, may use Dragoon with him to cast Rose Storm, but make sure his SP level is no higher than 1, or he will be killed fast.
Shana/Miranda: Have her attack, defend and use magic items. Her main focus overall should be healing, and similar to Lavitz/Albert make sure the SP level is no higher than 1, or she will be killed fast.
Rose: Have her use additions and heal every now and again. You may dragoon with her to cast Astral Drain, but make sure her SP level is no higher than 1, or she will be killed fast.
Haschel: Just have him use additions, he is not useful for anything else.
Meru: Have her attack, defend and use magic items. Her main focus overall should be healing, and similar to Lavitz/Albert and Shana/Miranda make sure the SP level is no higher than 1, or she will be killed fast.
Kongol: Just like Haschel; have him use only additions, he has no other purpose.
Note: Shana and Lavitz are included just in case a Game Shark or another cheating device is used to hack them back in the party.
Equip Albert/Lavitz with the Twister Glaive and have one of the party members use the Power up item on him, make sure the addition Gust of Wind Dance is up to level 5 and this boss will be a cake walk.
If you really want to use dragoons, just use this sequence: Raise the Dragoon Spirit gauge only to level 1, then use D-attack or the strongest D-magic. In the next turn they will revert to normal and the Dragon Block Staff can't reach the Dragoons.