Lets see.. what do I usually do

I usually have a powerhouse like Agrias master steal, I bring a group something to the effect of

Ramza with guts and draw out (kyomori and murasame help alot)
Orlandu with his swordskill
Beowulf with Chicken and Item subcommand (with Throw Item)
Agrias with Steal
Then either Mustadio or a random character with item/throw item, gun works nicely here

Equip everyone with 108 gems, having Excalibur on Orlandu helps alot. take out the assassins first turn before they can do anything, or at least kill 1 of them and then the next one the following turn. The faster you get the Assassins into demon form the easier the battle will be. Once Beowulf has chicken'd Elmdor down, use him and your 5th char to use x-potions to keep the group up from Elmdors area attacks.

Keep most of your group spread out, such that he will never hit more than 2 people with his area attack at a time. Once the demons are down, Ramza should be yelling at Agrias every turn while the item users keep whichever 2 ppl hit with Elmdors attacks healed with potions. You could opt to have Orlandu use items too but at this point of the fight he is useless, I throw him in the corner and let the other 4 do the work.

If you are getting horrible steal luck, it wont last long. After 10-20 minutes of the fight with Agrias being yelled at every turn, she'll begin stealing 2-3 times or more between Elmdors moves, making the fight go much quicker

(ps. take the shield first, makes it easier to steal everything else)