Ouch! Only 2 summoner/caster? How on Faerun did you get that far in HOF? Your cleric and wizard must have been working overtime.
Anyway...
Before battle, buff your party to the max. Cast stone skin on your tank. Strip your bard or your ranger naked so that they'll be the weakest party member (hopefully). Cast 4 animate dead on 6 oclock and 2 animate dead on 12 oclock. Pray to the gods and hope that lady luck is by your side.
Initiate battle,
Kill the 6 oclock summoner asap. If not he'll summon wave upon wave of undead. Next you should kill the wizard as he can unleash all the nasty spell on you. After that kill the archer.
Meanwhile, your tank should bait the meleers. Hopefully they should be chasing your tanks. If not they should be targeting your weakest party member (which is hopefully your bard or ranger). If things do go bad, cast Otiluke's resilient sphere on the character.
If your animate dead spell run out. Use Shade. If that run out use cacofiend. Or worst come to the worst. Huddle your characters on a corner of the map and repeatedly cast Gate and watch as hell minion's swarm the battlefield and make your computer slow to a crawl. Just make sure your characters have magic circle against evil spell on before you cast the gate spell.
OR
Some people mention that you don't actually have to battle them out for the sword. You don't need to anything. Just initiate the battle, grab the Cera Sumat sword and run away. They won't chase you that far.
But in doing so you won't gain the experience point from battling with them.