Mm; ok, it's been a long while since i played IWD2 (mebbe i should pick it up someday in a mpgame with some of you, it's a great enough game it is), but let's see.
I don't know the who's and what's of your party, but blah. I'll just give general advice on battles like this, since i only vaguely remember crushing them.
(jk, had a bit of trouble there too i recall 
)
So...have your fastest melee char(s) run straight to the sorcerers...even if they have pre-battle protections cast on them. We all know how i think about those: nothing a bit of melee hackin' can't fix!
Preferrably use weapons that do additional elemental damage. Sorcerer's shouldn't keep it up too long, moreover, in situations like this there's always one sissie sorc and one tough one. 
Ignore the harpies and elementals for the time being...summon some stuff to keep them busy. Keep ignoring the harpies all fight long, while you rain down on them with fire spells (weak point) if you get the chance. (i.e., if you're fighters aren't in the blast radius) Don't try using spells and/or bladed weapons on the earth elementals. they'll just shrug it off.
After the sorcerers, try to regroup a bit (in as far as possible ofcourse), and then make joint bash attempts on one earth elemental at a time. Have stoneskinned mage-types take their blows and have your blunt-weapon fighters kick their shiznits.
One by one...and then finish off the harpies. You might very well lose a man or two-three but blah, just raise them.
Alternatively...if you got an earlier unpatched IWD2 version
have your weakest char run straight into the enemy, and have one of your mages cast "otiluke's resilient sphere" on him/her.
He/she will be completely immune to all attacks/spells (except for dispel magic 
) and the enemies will all try to bash him/her. Watch how the sorcerers waste their best spells on the sphered char and then just unleash area spells hell. The harpies will fall, the sorcerers take damage at least, and then just whack the earth elementals.
)