I found the halfling mages tough, too. What I did was this:
1) Chaotic command on at least 3 party members (for my case, 1 melee fighter, 1 cleric and 1 mage since Minsc could use berserker rage)
2) Resist fear
3) Protection from evil
4) Defensive Harmony
5) Improved invisibility on druid
Then get druid to cast insect plague (100% spellcasting failure), while my own mages cast breach on the 2 most powerful halfling mages. Fighters charge up and whack halfling mages to pieces. In the meantime, cleric casts holy smite for all-round attack.
Warning: Some of the fights later in the planar sphere are even more challenging, but you'll find that out soon enough
